I've done one DBMS migration (MySQL > PostgreSQL) and also maintained an RDBMS agnostic system that supported MySQL, MSSQL/TransactSQL and PostgreSQL.

The system I migrated was extremely young and had terrible SQL discipline (I actually had to move queries using ye olde mysql_query to a tool set I wrote above pdo. This was a little while after the MySQL/Maria fragmentation and we weren't comfortable with Oracle stewardship but also didn't have high confidence in Maria's ability to resist further fragmentation.

After that migration we've committed hard to postgres and utilize a lot of language specific features where they're useful, I still encourage agnostic SQL where possible though.

I prefer writing raw SQL and, while I've added tooling to support Doctrine (an ORM framework in PHP), I don't personally use it. I also advocate against complex stored procedures, I personally think developers have difficulty understanding the whole system if stored procedures are allowed to be non-trivial - developers should be aware of the complexity that underlies their actions. I do, however, leverage materialized views extensively to keep a denormalized performant layer over our normalized source of truth.

I'm happy to answer any specific questions but it's super late here so it'll probably be in the morning.