SSH can be opened on opnsense, and so can cert based ssh!
Also a good idea is to enable serial access.
System -> Einstellungen -> Verwaltung
Proxmox can virtualize a connection, say from your windows/linux vm. Screen works wonders in Linux with serial connections. If you NEED troubleshooting on opnsense. With local AND Proxmox backups, I never needed that, but it’s still good to have…