39 second LCP? That's not an optimisation problem - that's architectural. Your theory about client-side rendering is almost certainly right. if Figma Sites needs a massive JS bundle to execute before anything paints, no amount of image compression or lazy loading will fix it.
The two thousand 404s are a separate but urgent issue. did you set up 301 redirects from your old Webflow URLs? if not, that's priority one - you're bleeding any link equity those pages had.
and no, fixing alt tags and H1s won't touch your crawling, indexing, or Core Web Vitals problems. That's basic hygiene. your real issues are a rendering architecture Google struggles with, broken URLs with no redirects, and page performance so bad Google probably can't render the pages properly during crawling.
Honest question - is moving back to Webflow an option? because fighting a nearly 40 second LCP at the platform level is a losing battle if the CMS itself is the bottleneck
