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Silver keys are not too expensive. 750e fits in with the $2.45USD price seen in some other prominent key-box games, it's even less if you buy energy in bulk.

The problem is distribution and use of box contents. The use of the rarity tier compounded with the user-unfriendly nature of accessorising is what is responsible for the natural deterrent you feel towards opening boxes. And you can't even trash them for a partial refund either.

Despite the game featuring an infinite inventory, they decided to create those damn accessory slots on gear thus creating premium inventory space. Oh, you want more slots? Buy more inventory space (costumes) or throw your existing items away. Needless to say, knowing that some accessories are rare and therefore impractical for most people to replace, it's only natural that you would think twice before using said accessories.

If they want accessorising to "flow" better, and they should as it's clear they think the art style is the games' major strength, then they have to rework the system to be more tolerant to the players wants.

1: Infinite inventory space.
That is to say, accessories are bound to the player, not their gear. With players able to freely wear any of their personal accessory collection at any time on any item without needing to future proof their choices, then this naturally creates a vacuum to sink a lot more accessories that they would never normally consider owning. It also solves the clipping/expiring problem, if you do away with blocking slots on certain items, then it's no longer a problem when something clips horribly or if the costume piece disappears in a month.
Notice this can sit alongside the current slot system if they implement a bridge of some sort - eg. An NPC, a craft machine, whatever. As it stands right now, if you have several valuable accessories you want to sell on a 'safe bet' helm/armor, then paying the gear unbind is cheaper than the accessory unbind - which could be viewed as cheating the system out of taxes from a certain point of view. To make player bound accessories favorable to the unbind system then the transition from gear bound to player bound MUST BE FREE OF CHARGE, you have to want people to do this for the bigger picture, taxing it just because you can would defeat the purpose. You don't have to nuke gear bound accessories out right though; while players with valuable but mismatched or duplicate accessories might feel cheated coming from an era of fighting with the previous restrictive system, if/when they realize their oddball/extraneous gear is now an asset to consume for themselves or sell onwards (paying only the gear unbind tax) then they can accept that the new system also has significant benefits for them too.

2: Simplify the style rarity.
Most prominently, downgrade Volcanic and Divine to be non-rare, or reduced rarity. These 2 styles might have been important in the past (most notably Vog and Skolver) but the keeping them rare means you also need to keep the appropriately themed costumes rare and/or POPULAR too. The guardians armor pack is affected by this, why should someone purchase the 2 themed costumes when it turns out accessorising them is an ordeal? Also, as time goes on and the popular choices of gear changes, theres no guarantee that these 2 themes will continue to be desirable in the future. Black Kat and Chaos are both Dusky; I don't recall any of the proposed gunner update sets being notably Volcanic or Divine either.
As for what should stay rare, Prismatic is an obvious contender, but the other one-time themes could also work. Perhaps make use of the recoloring system and offer random variations through the 3-color palate system, as long as the rare styles are special in some form and not seen in game common-as-dirt-gear themes (so this might also include Shadow).

3: Simplify the item rarity.
To complement the inventory expansion with player bound accessories, then the accessories need to be varied enough to justify box opening. If everyone is likely to get the same items over and over again, then the system will just go back to its old ways - but from artificial saturation to actually being saturated. Theres still room for rare items, but not so much for ultra-common ones. If new items also present themselves via drop boxes, then theres always going to be a reason to open, while still slowly increasing the overall rarity drop of pre-existing items through competition.

4: Item buy back.
Not necessarily needed, rather a sink to contain spill over from potential future item saturation conditions. No suggestions given, just make it worth while (not 1 crown). It doesn't even need to be money, as offering some currency payout would in effect be a refund on key costs.

I don't know about you, but I don't think a cosmetic system overhaul is only just an idea. I think it is inevitable. OOO has stuff to sell, but they can't do it if they won't empower us to buy.
Never say never, Accessory unbinding happened despite seeming like it never would (it could have been there since day 1)
You can't keep cramming an infinite conveyor belt of items into a system that isn't designed to absorb infinite items. This is one of the reasons people are complaining a lot every 2 weeks - they are seeing items entering the system but the system itself just does not work for them so they perceive it as pandering exclusively to the hardcore collectors, the only people who are willing to put up with the restrictions.
But-but my unique style! The cosmetic system has been in place for 3 years; flash sales are now a thing ready to push items into the hands of anyone who can consume it. Uniqueness is played out. Get over it.
Freely interchangeable cosmetics (ideally including personal color and height) is probably a pre-requisite for the often suggested "Costume Loadout" feature.

It was a shame that despite having a ton of different crafting materials, (some of them junk due to only being used to craft junk gear) the crafting system itself was short circuited with the introduction of boxes and keys; instead of making a game about combining all those little drops you find in the dungeon into new things, all that was pushed aside exclusively for looking for a single item drop (lockbox). A drop that asks "what have you done for me lately?" to boot.

I have a theory about this that I would love for SEGA to test. Silvery keys cost 750 energy, which is a pretty huge amount to many players, as opening boxes gets them absolutely nothing and will very very likely disappoint them. The average FSC run (which people mindlessly run because they think it pays best) nets you 11K in direct profit, counting the tokens you get. Silver keys cost around 50-65K crowns depending on how the energy market is feeling. If we take a common value from that range, say 56K (if energy costs ~7.5K crowns per hundred), that's literally 5-6 FSC runs down the drain for ONE new accessory that will very very likely make them lose out big time.

So what if, we adjusted the price of silver keys to be say... 200 energy, which is around a run and a half. You do a few runs, maybe you keep half of it and gamble the other half away. If you lose, so what? The thing cost you 200 CE. Instant gratification.

The best part of this, is that we only have a handful of styles for our accessories currently, and a large number of older items that are largely out of reach. This means that we can expand massively on what we put inside these boxes. Red + Blue as a style? SURE. Green + Dark Green + Yellowish? Call it leafy. There are thousands of possible combinations, and we only have like 6 implemented in lockboxes. We could also put costume armor and certain accessories in odd styles into the boxes as particularly rare drops- like a surge version of the valkyrie wings, or a lovely node slime receiver. This would create demand for the new accessories introduced and keep the value of boxes up.

Since, in the end it matters less the number of keys sold and more the amount of energy purchased for keys and because of keys alone, I could see this becoming a much much better solution to selling silver keys than relying on a handful of ramblin gamblin players that open huge numbers of prize boxes for the one big get.

ok sorry im late here, had to go to the infirmary, that wall of text really hurts if u run into it...

I like horses ideas of themed boxes and fehzors idea too, but maybe 300 energy would be a bit better

You don't see people farming for item drops (except for the Darkfang shield), why not give the keys a drop rate similar to items maybe a bit higher?

Or have a chance to get a key from 100% radiant forging (with a lower chance than forge box)?