SEMRUSH audit... how much does it help to get to 100%?

How much does fixing all the technical things on the SEMRUSH site help with SERPS?

Minimal. Focus on backlinks.

Not exactly, Semrush helps in keyword research and competition for that keyword.
Even after filling all technical things of site, the main source for ranking website is backlinks.
You cab optimize website in a proper format using semrush.