144 MB RAM usage would be a bit higher than expected, but not necessarily a problem - the continuously rising RAM usage however certainly is.
Given that there is quite a big potential of 'misconfiguration' (rogue logging, server processes filling up /tmp/ or similar issues), I would start debugging by having a look at the file sizes under /tmp/ and RAM usage of the process via (e.g.) htop, if you don't find a likely culprit rather quickly, starting over with a fresh configuration/ fresh flash and a minimal setup (just WAN/ LAN and the necessary VLAN setups, if applicable) and have a look what happens there for ~a week, before slowly enabling additional features one-by-one.
While the RPi ecosystem might not be the most pleasant one for developers, it has a strong following by OpenWrt users, so I'd be surprised if this was a generic issue for these devices.