alteration that isn't and can't be available for everyone
That's unfortunately how genetics works.
What is then the legal limit for doping of a trans male?
I thought being assigned female at birth made someone inherently and irreversibly weaker and so they would be non-competitive in men's sports /s.
Seriously though that's a largely solved problem. While specifics will vary depending on the specific org this set of guidelines outlined by the World Anti-Doping Agency is a decent enough framework and directly answers that.