You can get rid of the legacy bridge once all features are not using legacy code. Which might be possible but its not something that you can call upgrade nor migration. Its called refactoring
For front part you were able to do that already with ez5 and you can continue on ezplatform as well. If you have custom backed features that our Netgen Admin UI might be useful so that you can start refactoring those thing piece by piece. Check this for more info: https://www.netgenlabs.com/Blog/How-to-start-using-Netgen-Admin-UI
The structure of the database has changed very little, there are db upgrade scripts that you can run. Regarding the postgresql vs mysql, sorry, but can’t help much there. So far in our 12 years of using ez we had only worked with mysql, so I don’t know if the new stack is fully compatible with postgres. In theory it should as ez uses doctrine db access layer and doctrie support lot of different databases, but I don’t have any practical experience.