The query planners have diverged between MySQL and MariaDB; which one are you talking about? Both have received improvements in recent years, so the specific version is relevant as well. And at least both systems have supported index hints for decades (unlike another popular database that only very recently acknowledged their necessity in large production workloads...)

The locking anomalies are a well-known and well-documented side effect of InnoDB REPEATABLE READ differing from the standard in certain pathological cases. The real-world relevance is minor, especially since companies using external write-through caches are already making much worse inherent trade-offs in that area. Regardless, MariaDB fixed it, while providing an option to revert to the old behavior for MySQL compatibility. And fwiw this only comes up because MySQL/Maria defaults to REPEATABLE READ; yet meanwhile the overwhelming majority of Postgres users are totally fine with the much weaker guarantees of READ COMMITTED, because that's their default isolation level.

Semi-sync has some odd behaviors indeed; there are conceptual trade-offs that must be made if you want synchronous-like replication without the full performance penalty! Do you have some novel academic solution for this that you can propose?

Your last bullet is lacking in details so I cannot comment.