Based on your information, I believe PostgreSQL may not be a suitable option in this scenario. May I know any particular reason and your server information (CPU, RAM) why you stick with it under constrained scenario?

Given 200 MB of RAM with expected low number of connections, I believe it is your homelab and a good option would only be SQLite which benefit your SSD usage better. The issue of large INSERT is probably due to low RAM usage and even under idle, the PostgreSQL already consume a lot (at 64 MiB if I am not mistaken).

But let's say that you still want to stick with PostgreSQL then check if 200 MiB RAM is dedicated for PostgreSQL or shared with the OS one, even with alpine image, that already consume 64 MiB, then the amount of available RAM is just 100 MiB