Definitely going to be following this, as it's something I've thought of doing for my "next" phone - for basically all the reasons you've stated.
I'm sick and tired of not being in total control of my current phone.
I've thought about rooting it, but even if I did (it's a Samsung S7 from T-Mobile - which has a different processor and other stuff than the S7 that is easily rootable - and there are other "games" Samsung and probably T-Mobile play cat-n-mouse with the guys figuring out ways to root - which means a new update, etc - ugh all around ultimately), there's still the whole Google Store thing, walled gardens, etc.
Then there's the crap that happens with apps you like; I had this great file manager app that I loved, that also had numerous other tools - had used it for years, paid for the "ad-free" version, etc. Then one day, via HN or something, I don't recall exactly, someone noted that the app (and the company behind it) had been sold, and that the new version was doing dirty things behind the scenes, introducing bloatware and who knows what else - and people were abandoning it left and right. I recoiled - did some more research - and that night took it off my phone; I paid something like $15.00 for it - not a huge amount of money, but still - money lost. I ended up getting a different file manager, which is ok, but nothing like what I had - paid for the ad-free version - but who knows if the same thing will happen down the line?
Then there's the whole "file management", copying, sharing, etc angle - the whole BS about how in Android, if an app didn't create a directory, you can't put files in it (this apparently is conditional in some fashion - but it isn't very clear). This makes moving and managing files very difficult at times. Or the whole "let's not let users treat the device as a USB block device to copy/move/delete files from - and instead put in place another crappy layer that prevents all that - for some hand-wavy reason that didn't seem to be a problem when I first got my G1"...
There are other things, too - but you get the picture. I'm tired of it. Totally. My next phone will (hopefully) be one that I build, will run Linux, and will be under my complete control. I'm kinda hoping that by the time I get around to it, that there'll be an equivalent to the RasPi 4 in the Zero W's footprint. Add on a few other modules (IMU+GPS and 4G mainly) - and you have everything for a full phone. I might even not implement voice call features, as virtually everything I do tends to be texting-based anyway.
Don't want a keyboard, either - if I can get a decent capacitive touchscreen that'll connect to the Pi, I'll be fine; if I need a keyboard, bluetooth will work. Which also means I could use it as a "portable desktop" - all the convenience of a handheld, with the ability to become something more like a laptop on the occasion I need it (plus, if I need a big monitor, plugging that in would be easy too).
It's nice to see someone else blazing this trail - one that other's have gone down too (I know of other projects like this - some Arduino based, which is another route I have considered - but using an ESP32 as the core). You've already made a ton of progress, but I look forward to your future efforts. Good luck with it!