Proxmox will report SMART errors via email if you set that up. You could also run a system like Nagios to run the checks via another box. I actually run Home Assistant with the Proxmox HACS extension to monitor it. It’s on a VM so that isnt’ ideal, so I also run Node Red on the little I5 PBS box to send alerts if it can’t contact Proxmox itself now. The node going down without me realizing it was a bit of a wakeup call, though it failed my docker host and router over so seamlessly it was astounding.
I have nothing between the router and the modem except a switch so each Proxmox node can have a NIC on the external network and failover/migrating can pick up the modem and use it. I suppose I could VLAN, but the servers have 2 network ports anyway so that works fine.