that no single database connection is used simultaneously in two or more threads.
This will work fine with the existing actor model, as long as a connection is not shared by any two (or more actors). As long as at most one actor has the connection it will be properly accessed by at most one thread - the one the actor is running on. The connection type should therefore not be Sendable.
One could also make it work well in the single threaded, no mutexes at all mode I believe... If it really strictly needed "only one specific thread" and not just mutual exclusion, we could make this work once we get the custom executors which have an early pitch here: Support custom executors in Swift concurrency
but indeed, for now the multi threaded mode should be compatible with actors IMHO.