Thanks for your feedbacks !
Still experimenting with this, just to see what could be possible:
The GUI allows to make several marker operation (update/delete/add) in one row, with only one write request to the file (which is far quicker than having to make separate ones)
It is taking way more time than expected because of all the fixes I had to make, and the fact I'm not yet familiar with Python, which is quite hard to debug in REAPER. I may have to focus my work on projects which could help more people than this (LIST-INFO markers doesn't seems to be used by a lot of users), but it is already a nice proof of concept anyway