But you don't need to replace entire system to massively reduce surface area for the attack. Just do what was done for decades, and put the vulnerable devices between VPN and/or additional authorisation layer.

It's far less work than touching anything on controller side (at most changing some network settings) and requires attacker to at the very least penetrate your network perimeter and snoop around to find it rather than "run nmap on a controller's default port over country's IP range"

don't connect these to the internet that's how it was done