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Yes, there are a bunch of redis+disk projects out there, one big consideration is the size of your values: some must hold all keys in memory and only offload values to disk, while others (kvrocks, other rocksdb dbs) can offload both keys and values to disk, with some other tradeoffs.

Yes it is only limited by disk usage but it does have an in-memory cache.

I tested kvrocks about a year ago and don't have the benchmarks to hand anymore, but if I remember correctly kvrocks had a ~60% performance penalty compared to Redis, but depends on your disk speeds.

There are several redis-compatible disk backed solutions, https://github.com/OpenAtomFoundation/pika is the first that comes to mind but im positive there are others