What could be the consequences after the appearance of massive 404 pages on the site, due to moving to another engine?

Question

Search Engine Optimization

The online store is being transferred from the SaaS platform to its own engine. Since the vendor lock-in does not allow you to do the necessary functionality for business.

When moving, we decided not to transfer the URL of all products - almost none of them has a top position in the search results.

In this regard, questions :
1. Many pages from the Google index will become 404. products on the engine have new URLs that are different from those on SaaS. Doesn't it pose a problem? For example, the PS will decide that this is a gluing of a domain.
2. How can I make sure that new pages are indexed faster after the move? Sitemap, maybe something else?
3. Is it worth doing a redirect to the main page instead of the 404 page on the old URLs? Or it will not give any advantages?

Answer 1

1. There will be a dip in attendance. Because even old (missing) products sometimes bring quite a lot of visitors to the site in total. Catch them through 301. But it's better to drag 100% of the pages and only then experiment with 301, 410, etc.
2. Sitemap does not guarantee fast indexing in any way. If you do not change the domain, then it makes no sense to think about indexing already indexed pages. If the domain changes, external links help a lot. And a lot of links to the right pages. But here, with a new domain, you can also get a filter for an unnatural growth of the reference mass. In general, "the East is a delicate matter" (c).
3. You need to redirect not to the main page, but to the product category page (hub).

Answer 2

Alternatively,
1) collect all URLs of pages, for example, through Screaming Frog
2) filter those URLs of pages that collect traffic (GA, GSC) and redirect them 301 to the corresponding product cards
3) give the remaining URLs of pages that do not collect traffic 410 response
4) in the sitemap.xml, specify all new addresses that give a 200 response

Answer 3

kill seo
make redirects to the necessary pages, redirecting just to the main one will also kill