That RAM consumption is perfectly normal to me.
Generally, I’d upgrade resources when at least two of the following conditions are met:
- I Have a significantly higher number of users now
- My forum goes very laggy as soon as my users start participation in discussion
- My Host (VPS) is regularly crashing or going OOM or my service provider is nagging me about resources.
Higher utilisation of resources is a good sign while slowdowns are the first signs of real problem.