Why systemd did not restart your service
Restart= watches only the main process, so a dead child inside the same cgroup is invisible. How Type=, restart limits and the journal really work.
#cgroups
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Limit process memory and CPU with systemd
A capped process can still stall your whole VPS. Set MemoryHigh, MemoryMax, CPUQuota and TasksMax on a systemd unit, and read the OOM kill after.