A big part of it is the open source aspect, yes.
In addition, Plex is increasingly weighing down their offering with new “features” of questionable value. Some would probably use the term “enshittification” to describe the trend over the past year or two.
I bought a plex lifetime license a long time ago (2013), but for a newcomer I would recommend Jellyfin. You can also run them both simultaneously with no issue and decide for yourself.
For the 2nd point, see https://docs.docker.com/config/containers/resource_constraints/#limit-a-containers-access-to-memory