I think everything on the umbrel is in a container...but ty might look into this...
If you run the other processes in containers you can limit how much memory they can access.
Most people use docker for this. https://docs.docker.com/engine/containers/resource_constraints/
Discussion
I wonder if you can identify which cgroup(s) the containers run in and limit the memory of the containers that are consuming toouch memory.
https://medium.com/@kasunmaduraeng/docker-namespace-and-cgroups-dece27c209c7