Losing interest in SK, is there anything to do in it anymore?

Honestly SK is lacking in end game content, so if you really want to continue to play the best thing is to join an active guild with people you can make friends with and explore clockworks together. Focus on helping newer players while at the same time building up your inventory with all of the interesting weapons and armor- even if they may not be good, they can still be fun to use. Other than that there isn't much to do: the game is quite honestly a grind when you focus on getting everything so the most fun you will have is going to be playing missions with new recruits and also danger/shadow lairs with experienced friends. There's always lockdown and blast network as well; they can be a lot of fun.

TL;DR Try new things and meet new people

Spiral Knights is updated really infrequently, so we've had threads like this for years. You can find them by searching Google for things like "what keeps you playing site:forums.spiralknights.com" (without the quotation marks).

My first question is: Have you done everything? Have you done the danger missions? Shadow Lairs? Special event missions? Have you soloed them? Have you tried every weapon style? Have you mastered alchemers, pulsars, blast bombs, and Warmaster Rocket Hammer? Have you tried clearing levels using only troika charges? Have you tried using the same loadout with ASI+6 and with ASI-3?

The next thing I'd say is: I enjoy chatting with friends, helping novices in tough missions, collecting data on the game, etc. You might enjoy that too.

The next thing I'd say is: Maybe you should take a hiatus, do something else with your life, and return someday when there is new content (if ever). Cheers.

Start a guild. Get guild trophies that will keep you busy for a few years

"I enjoy chatting with friends"

You secretly have friends and I'm not invited? What is this Bopp? I thought you loved me.