Question regarding domain migration

Hello webmasters,

On 27 June, we did a domain migration, and everything went smooth for the first week. The website didn't suffer any downtime, and for the first week, the rankings were stable, and the whole traffic was redirected to the new domain. 

However, after a week, we saw most of our keywords thrown out of searches, and the traffic went down from 800 to 200 users per day. 

We expected that behavior, but we didn't expect it would take so long for the rankings to recover. The problem is that even after a month and a half, the traffic is still down, and most rankings haven't recovered yet. 

Now we are wondering if we did something wrong in the process, and our only guess is that we messed up by wrongly redirecting the sitemap. We found on the internet that this shouldn't be a problem, but I still want to see your input on this.

So the thing is, we redirected everything from the old domain to the new domain, including the sitemap. So at the moment, there's no sitemap on the old domain with the old links. If you enter olddomain.com/sitemap.xml, it will redirect to the newdomain.com/sitemap.xml that would have all the new links. Is this an actual problem? Should I disable this particular redirect and leave the old sitemap in place? If so, in which property should I link the old sitemap? To the property of the new domain or old domain?

To be noted that there's no error in GSC related to Coverage. On the new domain property, almost all pages have been marked as "Submitted and indexed," while for the old domain, most pages are shown as "Page with redirect" in Coverage. 

Other concerns we have that add more context to our issue:

- This is the strangest thing we have noticed. So before the migration, we had more than 300 ranking keywords, from which 30 keywords were in the top 3. After the migration, only a handful of keywords (like 5-6) have kept their rankings in the top 3, while the rest of the keywords have been thrown out of 100. Even at this moment, those 5-6 pages are still on top of the rankings and generate small traffic, while more than 200-300 keywords that previously had stable rankings are still out of 100, with few keywords bouncing from day to day. We would expect to see a slight and progressive increase in the number of stable keywords, but it looks like there's no big change, and still, most pages look like they have vanished from searches. How is it possible for Google to leave more than 300 pages out of 100 (pages that previously have been ranking pretty well) from a domain that sees it as good enough to rank it in the first position for other keywords? I mean, this whole time, those 5 pages are first on Google, and the rest vanished. This discrepancy is too significant.

- Google is very slow in crawling our website. At the beginning of the migration, most pages on the new domain were marked as "Mobile-friendly pages." However, after a week or so, we have started to see a slight decline in the number of mobile-friendly pages without seeing any increase in errors. So from 300+ pages marked as mobile-friendly, now, after more than a month, they are only 150. Our only guess is that Google is not crawling our website enough to get the necessary data for these metrics. Obviously, all pages are mobile-friendly and load under 1s. (We pass all core web vitals)

- We see strange behavior in GSC > Coverage for the new domain property. That is, we have an increase in the number of excluded pages due to "Page with redirect". This is because Google founds the version of the URLs that end with "/" for some pages, and those redirect to the URLs without "/", hence the issue. We have no idea how Google can find those URLs ending with "/" as we have removed them from sitemap and any other location. The number of affected pages for this particular error hasn't increased for a week or so.

- We are also planning to update our homepage. Should we wait until the domain migration is completely finished?

- Also, during this domain migration, Google has launched one or two updates. Is there any chance those updates can mess up the migration due to a faulty algorithm?

Any help is highly appreciated.

Thanks!