Of course, at some point of scaling needs a dedicated cache store will make sense anyway.
(Just some justifications from a german engineer :) )
(that being said, I try really hard not to judge; after all, i'm not without fault: it's 2021 and i'm using bash over cgi-bin to serve web pages for my own hobby projects :))) )
Isn't Redis an in-memory key/value store? Paying for RAM is not the same as paying for SSD.
If you're using postgres for caching only, as you do Redis, then you also do not need to provision block storage.
If you happen to already have Postgres running for other uses, you also do not need to provision block storage.
Finally, I would add that Redis clients such as Redisson are resource hogs that cause performance problems on apps, while pg clients are barely noticeable.
https://wakatime.com/blog/45-using-a-diskbased-redis-clone-t...