Big problem : planning not working as expected - not able to plan tasks

we have to maintain <100% global project affectation for each resource

This is not mandatory.
And maybe your issue come from fact that you try to maintain this constraint.
Resource can be affected 100% on several projects for same period.
Constraints will decide which will be first planned.

For instance, I have 1 resource 100% on 2 projects

You can split tasks and with dependencies get what you want.


Or you can also play with priorities...