This may be a stupid question, but why do you need OpenVPN any more, if
you have Wireguard?

Because it's already there? :)

Furthermore, some members of our IT team use macs (gasp!) and for them 
it would be much easier to continue to use OpenVPN than to set up 
wireguard-go.

I use wireguard on a Mac and it was as simple as "brew install wireguard-tools"; create config; "sudo wg-quick up wg0".

My even more stupid question is "why use wireguard if the only thing it's carrying is ssh?" - but I guess it's a convenient way to tunnel to a network which doesn't have public-routed addresses.

(Aside: I wish ssh had a feature like SNI, so that you could build an ssh proxy that forwards incoming connections to the right host.  I have done this before using an inbound SOCKS proxy, but it's messy to use)