Most efficient way to migrate Themes

What would be the most efficient way to migrate a WordPress website that is heavily themed (pagebuilders etc) to something a bit more minimalistic like GeneratePress.

I have looked at just switching and fixing what needs to be fixed, but that's pretty much everything.
All 100+ pages & articles.

At this point, I was looking at rebuilding the site from grounds up and mimic the current URLs, articles and everything inbetween.

The website is a year old and is already ranking.