It took me years before I started tuning the memory-related configuration of postgres for workload, schema and data, in any way. It "just works" for the first ten thousand concurrent users.
Well, most people working on a car don’t have a car lift: it only makes sense when you need to safely work on a large volume of cars. If you only work on one or two, a jack and a pile of wood works just fine.
Modern JVMs are pretty effective in most scenarios right out of the box.
I just tend to use https://github.com/le0pard/pgtune