Why not use Tailscale on each device?

No need to expose any ports, no need for a bastion, no need for any complicated method of retrieving their public IP address, can use ACLs to restrict their access to other devices on the tailnet (if they're tech-savvy enough to go looking at the tailnet in the first place).

Essentially, as long as they have internet and Tailscale is running, you'll be able to connect to their device without exposing anything over the internet.

Any reason you can't use a locally hosted VPN? That would be my solution for something like this. Either use tailscale or use a wireguard VPN and a dynamic DNS service.

Later on I might consider adding some PiKVMs in order to be able to more safely reboot/troubleshoot/access BIOS.

That is a lot of effort to go through to avoid using a VPN.

Yep, a bastion is what you're looking for. I use an rpi + a Dynamic DNS record in a script on the pi to automatically update firewall and ssh rules if my IP updates. Of course, you may need to do some configuration depending on their network setup.

Something like a raspberry pi or equivalent, and use reverse SSH set up to connect to a server with a known address on your end.

This means that ports don't need to be opened on their end.

Also if you go with a gateway host, shift SSH to a randomised port like 37465, and install fail2ban.