Your plan about the level up system sounds really good. That is such a cool diversion from Undertale's static difficulty and sounds very in-line with modern indie games. That idea sounds exciting to me.
The cursor disconnecting happens during fullscreen as well. It is triggered by moving your mouse physically very fast aka faster than the catch-up mechanic, and then stopping all movement completely. I could record it but it's pretty much as you would expect. It's agnostic to Windows, Linux, and browser.
Edit: I would like to add my touchscreen experience was on a Chromebook Duet Gen 9 which is a low-spec but 11" screen device. I would imagine it's harder on phones due to the smaller screen but I think a dedicated player could make anything work.
Edit NEW: if you are reading this, I am unable to post new comments because of a shadowban on this site or failed Cloudflare Turnstile implementation.