I have spent years fixing WordPress sites bogged down by technical debt. You do...

https://spark-wiki.win/index.php/Technical_SEO_Audit_for_WordPress:_What_Usually_Breaks_(And_How_to_Fix_It)

I have spent years fixing WordPress sites bogged down by technical debt. You do not need a big rebuild to boost rankings. Start by adding image compression for speed, adding internal links to old posts, and killing spam with Akismet