I've been using UpStash's serverless Redis offering and it's worked super well for my needs. Scales to zero/free which was nice for getting started, and using their http SDK didn't need to worry about concurrent connection limits when calling from simultaneous cloud functions. & not a second of downtime in the few months I've used it so far.

Want to move more of my app's datastore to Redis now that I've learned more about sorted sets etc.