How would caching on the db layer help with your web service?

In my experience, caching makes most sense on the CDN layer. Which not only caches the DB requests but the result of the rendering and everything else. So most requests do not even hit your server. And those that do need fresh data anyhow.

As I said, my app is write heavy. So there are several separate processes that constantly write to the database, but of course, often, before writing, they need to read in order to decide what/where to write. Currently they need to have their own read cache in order to not clog the database.

The "web service" is only the user facing part which bears the least load. Read caching is useful there too as users look at statistics, so calculating them once every 5-10 minutes and caching them is needed, as that requires scanning the whole database.

A CDN is something I don't even have. It's not needed for the amount of users I have.

If I was using Postgres, these writer processes + the web service would share the same read cache for free (coming from Posgres itself). The difference wouldn't be huge if I would migrate right now, but now I already have the custom caching.