It's been a long time since I've looked at KeyDB, but IIRC KeyDB is just Redis plus a spinlock. It's actually still very performant. There are other "toy" reimplementations of Redis in Rust that take the same approach and aren't even as performant as single threaded Redis.
The next approach you could take is using something like Glommio and take a thread-per-core design to Redis. I think that approach has a lot of potential, but the design becomes more complex (you now need something like distributed transactions for "cross-core" Redis transactions and mutli-gets)
"ScyllaDB also handles complex query processing in the same engine. RonDB does all complex query processing in MySQL in a separate program." — i.e., as soon as things might get a bit hairy, RonDB punts it off-system. While a clever way to keep performance numbers high, you'd have to also then analyze the MySQL latencies and throughputs to get an overall view of complete system performance.
It's a clever dodge, but it's not a magical way to eliminate the need for those pesky CPU cycle times.
The next approach you could take is using something like Glommio and take a thread-per-core design to Redis. I think that approach has a lot of potential, but the design becomes more complex (you now need something like distributed transactions for "cross-core" Redis transactions and mutli-gets)