Is upgrading to python3.8 is possible in Leap15.2 without stability issue and conflict with other part of OS?
If yes, should it be done via factory repo?
I have the same request for upgrading gcc to 10.
You can install python3 3.8 from the Python Factory repo,
Here is the one-click install
My experience has been that although there always some risk with installing anything from a Factory repo, it usually is without incident.
Installing multiple GCC on the same system side by side works without incident.
The GCC itself is a binary that can even be copied between machines, there is nothing about it that embeds in the system.
You can install using a package for easy management and possible future removal.
However you install each of your gcc, it’s useful to set up an Alternatives to easily switch between each gcc, and I wrote a Wiki article how to do this