Can't you just add something like ServerAliveCountMaxto help with solving stale connections?

So something like that would solve that

[Unit] Description=look ma, no autossh After=network.target

[Service] Type=exec ExecStart=/usr/bin/ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 -o ExitOnForwardFailure=yes -Nn -R 7070:localhost:22 pc 'sleep 20m' Restart=always RestartSec=20 RuntimeMaxSec=30m

[Install] WantedBy=default.target