I have a BOOX Max Lumi, it has all those features, for free. Plus is a fully functioning android tablet.

Not sure it's any open (Kobo devices usually are hackable), but Kobo Elipsa[0] is feature par with Remarkable 2.

[0] https://us.kobobooks.com/products/kobo-elipsa

I'm being a bit future-looking here and considering alternatives to the gen3 reMarkable, not the current hardware. Specifically, the PineNote seems like it could fill the niche for me.

The Onyx Boox series come in different sizes and are just Android with a custom launcher, and they're cheaper and with more choice (most notably regarding screen size).

Caution however, the company violates the GPL, which might be a deal-breaker for some.

I got a Supernote and its a bit cheaper and has really great features. Remarkable shies away from anything but writing, so sync is limited - while the supernote has great calendar sync thats come in handle for note taking.