In no particular order, here's the ones I've been following:

-https://postmarketos.org/

-https://puri.sm/products/librem-5/

-https://www.pine64.org/pinephone/

I'm the most optimistic about Postmarket in the long run, but the others are actual hardware so there's that.

Since you say you've been following it, any idea what's going on with modems on postmarketOS? Is there any chance of getting working calls/SMS/data working on conventional phones or is that only likely to work on open source stuff like PinePhone?

I think I've laid the foundation for supporting Qualcomm modems with ofono (committed to the pmOS repos.) My device has a vendor kernel, and I helped someone else with a mainline kernel. I haven't seen much movement other than that, maybe due to lack of interest or skill in potential developers. (Each device needs a few custom bits like firmware packaged.) Another problem might be a lack of usable GUI to actually use the modem features, but I'm not up to speed on recent progress.

What backdoor? The flaw described uses a user-installed malicious application to exploit a bug in the permission system (which also must be user approved). I dont see any claim or evidence of a manufacturer backdoor.

Yes, I'm sure a GNU/Linux phone will have fewer bugs... hahahaha