I didn't have particular time range, the style was popular from 1000s-early 1600s. I just searched for "illuminated manuscript" and"marginalia" (the doodles on the edges of pages by bored monks), and then used the library references to find scans in public collections to flips through until I found one I liked.
I didn't search much outside of English (a bit of Google translated Dutch I think), so if you're in Bavaria you probably have more access than I did. it might be worth finding out the equivalent terms in german and to search any state or public collections that have been scanned.
For animations, I just went with a monty python / south park style where I just transformed the whole layer. Depending on the game it might work for you
You can do color filters to try to unify the colors of different assets too (there are a lot of yt tutorials)