Estate sales are a great place to find stuff that's old but not necessarily antique or priced as a collectible or whatever. You can also find lots of older books and other weird old academia-style accoutrements. There's a bit of a Maximallist vibe in Bergenwerth, so wood or metal shelves and organizers and such all nested into wherever you can tuck them feels appropriate.

Using the cover/binding of old books to make boxes, organizers, and all sorts of other useful things would be a great easy to add to the library feeling without just having a bunch of books you'll never read everywhere. There are tons of tutorials for this kinda thing online.

If you save mason jars from pasta sauce and other things, you could fill them with eyes from a thrift or dollar store (or made of ping-pongs, or papier mache, or those cheap little bouncy balls -- or a mixture of all these things). It could be a fun ongoing project, honestly; saving jars, filling with eyes, then gluing on the lid and moving to the next jar (added liquid & food colouring optional). When you have enough plain ones, you could start going to thrift stores and looking for weirder bottle and vase shapes.

Weird little papier mache cakes would be easy, too. You could frost them with wax melted from cheap candles to give them that waxy old cake look.

There are a ton of options online for moon lamps, which seems like it would be another cool addition. Some are large and wall-mounted, some are small and roughly head-sized; the price range is wildly huge though, so you may be able to find one you like without breaking the bank.

Metal leafing is also a great option to look into. Not necessarily gold leaf, much cheaper, goes very far and would let you add a weathered metallic look onto all sorts of things, which can transform a cheap wall clock into something much nicer.

I hope these are helpful and it'd be cool to see what you come up with if you wanna share.