A lot of the secure applications I use have a linux app of some sort (DEB, AppImage, Docker) but I fail to see how any of that is going to work on a phone.

I certainly don't see how a docker container would be good for battery life on a phone either

ProtonMail - for example - on Linux, requires a bridge application in order to talk to a IMAP capable email client.

Etesync requires a bridge in the form of a docker image.

Wire has an AppImage that I'm certain won't work well on a mobile screen. Signal has a DEB for Debian based environments and I doubt that'll play nice on mobile too.

The only real way to do a Libre phone is for me to invest entirely in the Libre ecosystem for mail, chat, VPN, everything.

if you've spent time moving your stuff into more secure and diverse sets of applications and services, and what's more - convincing friends and family to communicate with you over those very things, you'll have to migrate once more in order to use this phone.

I would bet money that Firefox doesn't even work on it in a mobile/phone-mode. You'll have to use their stock browser which will probably be based on Epiphany.