"• It should support a stylus, so highlighting text is easier."
Everyone should start with reading with understanding first, annotating only afterwards
To my knowledge, the software where scribbling solved, the academic reading is rather unsolved...
For 200$ you should stick just for the reading part.
At least 8" e-reader is recommended (for 200$ it can be a used one), on which koreader installable.
Koreader supports minimalist file structure, showing file names only (that is really fast) without the sluggish art covers and unnecessary screen updates, then the browsing gets very quick.
You can define gesture for history & favourites, too (rather essential).
PocketBook stock software also supports such a file view, however its speed is horrible (you need to pan or very slowly page turn, then the sluggish screen updates kick in).