Thank you johnh, that was my third option « go to the src » (btw you are right, better to use dev that modify in place package). However, this means I loose automatic upgrades when a new release appears.
The Overrides.toml seemed better suited as a local installation answer, but I do not understand what syntax to use, even after reading and re-reading Pkg doc … may be I should read again? I was unable to find working similar examples by googling, so asked for help.
Thanks anyway, will keep you updated if no new suggestion appears.