The difficulty is largely fine imo. Maybe have them deal a bit less damage and have a bit less normal defense? That way normal weapons could hit harder and armor could be useable.
Shadow lair keys should cost special materials that drop in the arcade during elite mode, on T3 danger rooms. They would optimally cost a good 10ish eternal orbs. This would prevent new players from doing them until they are established, restrict older players to doing a reasonable amount of runs and not expecting the lairs to last forever, and lower the cost of entry so that they aren't as pay to play necessarily.
The reward structure is bad because the old shadow lair materials are common because of people farming them. Ergo I propose 4 new materials to drop, advanced versions of the 4 existing materials. These would drop instead of the current with around 40% odds on advanced and guaranteed odds on elite.
The new SL mats would then craft trophy weapons (that require 10 of a single new mat) that are useable and decent, but most importantly, would cost a lot of energy (10K ceish), orbs, and materials. If you're feeling generous you could even let them craft a F2P mixmaster variant or something, but this is likely unnecessary. You'd see someone with one of these weapons and you'd know how amazingly gamerific they were for beating the Shadow Liar. None of the materials and weapons you could get would be tradeable.
I do have a bunch of fun ideas for hard to get weapons and gear.
Rabid Green Snarbolax Set (per piece)
-Negative poison resist high
=Otherwise same as snarb armor
+Sword CTR: Medium
+MSI: Low
Rabid Green Snarbolax Shield
=Same defense as snarboalx shield
+Sword ASI: Med
Rabid Green Snarbolax blade/briar barrage
-Less damage
+poison, very high proc chance
Phantom Blade = Fang of Vog, but curse and shadow
-Charge curses you like fang of vog's fire
+It's epic, like fang of vog
Almirian Spear = Shadow or Elemental Flourish
=Same as flamberge, but it's shadow. Or elemental?
Fire Blitz! = Fire variation of blitz
=Same as pepperbox, but it's piercing. And awesome.
Third Troika/Shock Troika
+It's sudaruska, but shock? I think this is already in the game according to the wiki?
MSI Buckler
+Swiftstrike stats but
+2 MSI instead of +3 ASI, Maybe +3 MSI?
+Maybe make it 4 star?
Ghostmane Stalker Sawblades
+Elemental Cutter? If that's an option?
+Otherwise divine avenger variant that hits faster but has a worse charge?
I could go on but I won't bore you to death.
Some very nice ideas
the only ones I think are not good is the +2 or +3 msi shield because it would make all other shields obsolete and that would be a shame
the other one I'm not 100% sure about but having piercing+elemental+normal variants of DR is probably a mistake. First of all the normal variant would be useless because the damage would not measure up to the specialized ones, you would basically never use the normal variant when you have access to a specialized variant of DR. The 2nd problem is that it will be very hard to justify using any other damage bomb over an elemental + shadow DR and then probably either piercing DR or DBB / Rabid DBB. So then I don't know if it's a good thing that bombers will only ever use DR variants to deal damage, maybe that's fine because they still have vortex and mist bombs to fulfiill other roles though. If I had to take a middle ground there, I would go for adding just an elemental DR and leaving it at that. The normal one isn't useful, the piercing one would probably only get in the way of DBB and would definitely makes greavers a joke when they're supposed to be a somewhat challenging enemy for bombers (forcing you to use barrier pickups, sprite abilities or straight up bringing a WRH/other weapon to counter them)