The performance issues are due to the candlepin issue which has been discussed in length in Feedback for Foreman 3.11 & Katello 4.13. So that is being solved.
Regarding the database upgrade issue, I would say RedHat has to fix their upgrade script to handle this case properly. However, I don’t have this issue and I find it difficult to file a bug with RedHat to fix this or Foreman to put in a proper detection or mitigation.
I haven’t seen anyone else with your locale problem (yet) so I guess unless it turns out a major problem for many more users you’ll have to figure out yourself how to make sure the new database uses the correct locale.