PostmarkedOS is a bit frustrating as it makes you jumping through many hoops if you want to install anything not 100% FoSS (due to alpine base)

I personally think arch and mobian are the best options right now, using the phosh UI (I like gnome-mobile as well, but it is not as mobile friendly as phosh)

Now to the hardware part, you have basically two options:

1. Use a phone where most drivers are in the mainline kernel, but with bad performance due to weak SoC (e.g. pine phone pro)
2. Use a phone with strong(er) SoC but relay on custom kernel from the phone brand, locking you in place of the version of the kernel they modified (e.g. oneplus 6, I think)

At least this is how I remember it

When we come to App support; you mostly have to rely on webApps for proprietary services like train ticket, uber, etc.

For massages, I would recommend a Matrix server and the various bridges to third party messaging services that exist for it (be aware that some services consider a connection via bridge as incompatible with their ToS and may block you)