Can you help me out and post a one-liner I can test with?
I believe the web-site errors are indicative that the db password for the new VM should be different from db password of the old VM.
When I change the db password in WP config to the db password of the old VM I get an error that the connection to the database cannot be established. Isn't it the confirmation that the old db password is incorrect?
Also, since the export on mysql 5.7.23 and import into mysql 8.0.16 is not working properly, can you test if I can upgrade the mysql 8.0.16 on the snapshot of old VM, then create an export file from 8.0.16 and import that file?
I'm volunteering for the local non-profit with this, the reason I went with your stack is to avoid things like having to research and spend time during upgrades.
I badly need some tested instructions on how to migrate from your Multisite Wordpress 4.9.8-0 to your Multisite Wordpress 5.1.1/5.2.