Lost almost all rankings 2 weeks post domain change (site migration) - any idea why?

Hey, I lost basically over 90% of my traffic due to extensive loss in rankings after i migrated my domain to a new site for a domain name change. I was pretty sure I followed all the rules and steps.

- I used a regress 301 in my httacess file since none of the urls changed. 1-2 days after the migration everything looked alright, about 50% of the rankings were transmitted. but from then on everything went downhill and keeps getting worse.

- I was able to change the links and anchors of my most important backlinks to point to the new site, although these have not been registered by search console yet.

Possible cause: Should I have changed the internal links on the old domain to point to the new site as well? I fear I could have lost authority due to this. I thought the wildcard redirects from the httacess code would be enough. Apart from that I followed all the steps correctly.