You're right on the QML ; I'll admit I didn't look super hard, I was under the impression QML eventually compiled down but maybe there's some interpreter-type event loop implementation that results in a slower than expected UI.
Based on this Stackover flow post I guess it CAN be compiled but that depends on the version, open-source, commercial, etc:
JMP.chat exists but honestly that's kind of a crutch or work around if your primary cell phone can't make or receive calls on its own without ANOTHER broker or provider.
One thing I did like about Sailfish OS and to a lesser extent Ubuntu is that it's mostly "standard" Linux under the hood so you can do things like run dnsmasq or something like that and tweak it as needed.
But in this day and age I think the bare minimum for a cell phone is calling, texting (SMS), possibly MMS and GPS/maps that don't need a network or Internet connection. I'm hoping PostmarketOS, Sailfish, Ubuntu Touch, open-source-smart-phone will get there but I'm not holding my breath.
I heard rumours that the new nokia 3310 5g which will be released in may will run sailfish os. Will be interesting to see if it is true, if it will be the same mess that kaios was on the nokia 8000 or if it will actually work well.
VoLTE is hard but JMP.chat can act as a workaround.