You can set or increase the swap file in your Pi OS to increase the perceived amount of memory. InfluxDB will not know the difference but swapfiles will be slower.
I addressed some of your other questions around degradation over here in this post: Out-of-memory on backup + queries
Because you are on a Pi, adjusting down the TSM cache size will free up some memory for other uses. It has a high default for a Pi ~1gb.
Do you need only up to 30day query periods? Do you need longer periods?
