I use Librera it has option to cut pages in half (left-right), bookmarks, auto-scroll, change of colors, define app to use for translation of marked text, etc. IMO best reader there is
rpg
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This sounds exactly like what I want
I've got a kobo libre color and it's decent. Not the size you're looking for, but it's eink 
Pretty good for reading rpg full color pdf's and stuff? Not too slow or anything?
EDIT: The picture just loaded for me. That doesn't look too bad.
It's yellowed because it has nighttime settings on. It's quick enough for reading
In my experience reading PDFs is a really light requirement so you can do it on otherwise ancient hardware. I've got an old nexus 9 which is painfully slow for web browsing or games, but I use it for the exact situation you need, reading TTRPG books.
Save some ewaste by digging out an old tablet or picking up one for a few quid on eBay!
How expensive is "too expensive"? I have a boox note 3 and read TTRPGs all the time on it. Their color eink screens are really good. You can get the go series for 200~250-ish
https://shop.boox.com/products/go7
I can post a picture later today (busy atm) of my boox with a color PDF if you want to see how it looks
I'd love to see that. 200-250 doesn't sound too bad.



note that this is the air 3; its a bit more expensive than the go, which is the ~250ish model i mentioned above. The go is also only 7 inches instead of...i think 12 on this? They use the same sort of color eink on all their tablets, though

My amazing iPad does this well. You can get a 10th generation starting about US$250. ALSO, look out for November deals.
How big is it? I'd prefer like 10-13" if they get that big.
The iPad has a variety of sizes up to 13”.
I use an iPad Pro 10" mostly for my ttrpg PDF reading. I also have an Apple Pencil and I use an app to take notes, write the campaign, manage my character sheet. The pro is overkill for this, and if I was getting an iPad for I would go with a normal iPad, which has an 11" screen on the current model starting at $349 USD for the current model.
If I did not already have an iPad, I would get one of those color eink readers talked about elsewhere in the thread.
Honestly nowadays, even with fancy pdf, my 200€ Xiami does it. It'not 2015, even simple tabs have a lot of ram
I bought an iPad Pro 12.9 inch (IIRC) back in like 2018 or 2019. If you can get an old second-hand iPad I'd highly recommend it. It's fast enough to be really smooth even on very large heavy PDFs, and it's nearly A4 size, so the reading experience is excellent.
But bought new it's insanely expensive. I really wish companies would separate screen size from power. Not everyone who wants a big screen also wants the highest of high-end stuff, damn it!