300 of 400 product pages stuck on crawled currently not indexed for two months Technical

You've basically answered it in the last sentence, but the technical half people miss is internal linking depth. Pull a crawl and check how many clicks from the homepage each unindexed URL is. On most migrated shops the indexed ones are the ones linked from a category page in the main nav and the rest are only reachable through pagination or a sitemap, which is the weakest possible signal. Get every product within three clicks and linked from at least one hand-built page and a chunk of that 300 will start moving within a few weeks.

Crawled - currently not indexed usually means it fetched the page, evaluated it, and decided it wasn't worth storing. Duplicate manufacturer copy across 300 URLs is the textbook cause. You don't need 300 essays. Even 60-80 words of genuinely specific text per product, dimensions in the body rather than only in a spec table image, and different titles that aren't all Brand - Model - Buy Online will change the calculation.

Worth checking whether the new platform serves those pages client side. Fetch one with curl -s <url> | wc -c and then look at what's actually in the HTML. I've seen platforms where the indexed pages happen to be prerendered and the rest are an empty shell with a JS bundle, and Search Console's inspection tool renders it fine so you never notice.

One boring thing: confirm your April migration kept 301s from every old product URL. If a page lost its history it's effectively a brand new URL competing with nothing to its name, and thin new URLs are exactly what gets parked in that report.

Don't submit them for indexing one at a time, it doesn't do what people hope and you'll burn a week clicking.