I’ll add to your list:
0. install TWRP recovery, so I can backup and restore full images of OS I installed.
For installing Ubuntu Touch it is really not difficult, just download one of the installers (UBPorts installer or CPT) on your computer, plug your phone with a USB cable then follow the on-screen instructions
Thank you very much, you have convinced me. FPOS I’m not going to use it, since it seems it’s just FPOOS, but with Google apps. I’m not very close to Google …
After FPOOS, I will use LOS. Then FOS, Ubuntu and finally Sailfish (the system that I really want to try). If Sailfish is not my definitive system, I will return to FPOOS or Lineage (I do not know which of the two).
Stay open minded and give Ubuntu touch (and Firefox) a chance. Otherwise, why testing them.
And I would at least take two days for every system to test it and do some more in depth checking of how it suits my needs. Especially when you are new to it and have to get used to it first.
Btw: Just by what I read on the forum, you might run into troubles on changing from one OS to the other, that render testing two in a day virtually impossible.