stable releases vs test releases vs ?

[mudflap]

The only real difference between Manjaro's "stable" and "unstable" branches (forget about "testing" in this context, I don't think Manjaro-ARM has a testing branch) is that updates are delayed on the stable branch, unstable gets upstream's releases pretty much immediately when they're released. That being said, not even unstable gets "pre-release" versions of software, all the packages are what upstream themselves release as "stable".

My real question is, though, can't I just add the (excuse my ignorance) "ppa" source to the "Discover app" for whatever the "not quite bleeding edge" release is, and have that be my default build? or does it still need to be done via SD card / flashing / terminal commands? 

You can switch branches without flashing a nightly image, but it does involve using the terminal on the PinePhone, see Manjaro wiki on switching branches.

Code:

sudo pacman-mirrors --api --set-branch unstable

to switch to the unstable branch and

Code:

sudo pacman -Syyuu

to install updates from the unstable branch; -yy is to force refreshing of the database, -uu to allow downgrades in case pacman incorretly interprets the version of a newer package as being an older one which does occasionally happen.

I think I'd like to be a little more involved in testing, but not sure how dangerous I want to be. 

It's not much more dangerous than stable: I've experienced mess-ups from the Manjaro devs or general (upstream) regressions make their way to stable too, but the fixes come quicker in unstable.

This is exactly the response I was looking for! thank you so much.