The semi-vertical ones help compared to a standard (horizontal) mouse, but as someone who had a lot of pain, they were not enough for me. For example, I had the Microsoft sculpt mouse, which came with their split keyboard (the main reason I bought it) and still experienced pain. Then I went more vertical to the Logitech Lift which was better, but did not eliminate the pain entirely. Finally, I now use the Evoluent Vertical mouse and now no longer experience pain in my mousing hand.
The only drawback is when I used a standard mouse (totally horizontal), I used to switch hands and use it ambidextrously. Now I obviously can’t switch hands with these mice.
I think for someone who doesn’t experience persistent pain, a semi-vertical mouse may be well enough.
Boox has 10.3" and 13.3" models: https://shop.boox.com/collections/all-products/13-3 The 13.3 would be closest to A4 (assuming that’s what you meant, A4 or 8.5x11). They run Android, you can install other reader apps like Kindle, Kobo, Libby. You can load PDFs by USB (which I do). There was a browser interface which I’d tried and it worked, but I just don’t use it. No idea if it’s still available. The built-in reading app can mark up epubs the same as PDFs. I purchased a Wacom stylus that has a side button for the eraser function because the original it comes with has the eraser on the top. I didn’t like having to flip the stylus over.
I have a predecessor 13.3" model, the Max Lumi, which is no longer available, but I am still fairly pleased with it. I’d gotten mine for the purpose of splitscreening a document on one side and notebook on the other side, to take notes for school. If I were to buy again now, I would go for the slightly smaller 10.3 size to be more portable, but unlikely to use spiltscreen. I would also opt for colour (which was not yet an option when I bought mine.)