One problem with almost all openworld-crafting games is, you need to decide yourself what you want to go for.
You asked for bigger bosses but also said you are equipment is already maxed. If they were bigger bosses, you go there, kill them... and then? Asking for even bigger bosses?
You want a boss almost impossible to kill? Until anybody posts a cheesy way to kill it anyway, then you go with this. Like so many people building cheesy horde bases and then complain about there is no challenge anymore.
You want to survive radzone? Once you got your suit you easily survive the radone. And then?
Set your own targets to whatever you want or interested in.
If you on day 100 and are bored... increase difficulty level and with that your gamestage increases immediately. Survive a gs2000 horde instead of just a gs1000 horde.
Or maybe do tier 5 trader quests, at least in every tier 5 able building in your own town.
Make it a rouge-like game. No base building, try how long you can survive (maybe even with permadeath).
Try nomad style, maybe with building but never use a base twice. Try how long you can survive that way.
Make exploring the whole map your target. Without using vehicles!
Try other things. Maybe prepare a skyscraper. On BM stay on top until the horde is inside, lock them, then let the whole skyscraper collapse (the skyscraper has to prepared to make the Zs go in and up and you need to bring it to final collapse fast).
Play a game without gardening and don't buy food from vending machines or traders. With no loot respawn (and not doing fetch quests from traders to get free loot respawn).
There are many threads here in the forum discussing about what could be different challenges.
Dishong-Tower is a mod afaik? If it is still to boring just getting on top of the tower, try to speedrun it.
Ah, there are dozens of mods you can try anyway.
In the end there is always the question left "what's next?".
If you are still bored... well for most people any game becomes boring after playing it for a long time. Then move on to another game.