It’s not recommended to modify profiles in plexmediaserver/Resources/Profiles. PMS will replace your changes with the default profile. You need to create a new folder (I have a QNAP but probably other Linux versions have the same path, if in doubt you can check the path for Logs, should be the same for Profies) located in plexmediaserver/Library/Plex Media Server/Profiles and copy your customized profiled to this new folder.
In case you have an Intel 64-bit NAS the newly released HW-transcoding can offload the CPU significantly. If you have an Intel 32-bit you can rely on the option to “optimize” your media (some ARMv7 and ARMv9 can do it as well).
If your Plex clients can be parametrized for lower bitrates than this may be your best option. If the kids change it they will suffer (stutering and the like), so they will learn to not do it.