I can't speak for the BOOX Tab Ultra C directly, but I'm interested in opinions on it, because I'm thinking about buying one as well.
I have the BOOX Note Air though. Overall, I really like it. I use it to read books and learning material, do math exercises and occasionally to draw. I love that it's smart (full Android, download any app) and dumb (slow, grayscale, don't get distracted easily) at the same time.
Technically you can use it for browsing the web, however I wouldn't recommend it. It' too slow if you want to hop between different sites. It's only comfortable if you access a specific site to read on for a while. I hope the BOOX Tab Ultra C is faster (I mean it should), but if the web is one of your main use cases, I'd rather go for a proper tablet. Even if the ePaper screen is relatively fast, it's still too slow to be fun. Since you're already experienced using ePaper screen, I'm confident you can judge that for yourself though.
My motivation to get an upgrade is mainly the Note Air quickly slows down when drawing anything semi-complex or above. I can recommend it for reading, taking notes and (limited) drawing. For anything else, I recommend a tablet instead.