I was wondering which versions I should try to port in.
1.7, 1.8, 1.9?
Good to see you "back in business"!
Bringing in more than one version is OK, but what are people using the
most? Which ones are approaching end of life/support?
This one is easy: 1.9 is the one you should be porting. Both 1.7 and 1.8 will run out of community support in the course of 2022.
As far as the optional features, I can just leave a message after
installation about being able to get those features afterwards by
downloading the necessary packages.
Sounds like a sound approach: get the primary dependencies into OpenBSD first and work your way through the optionals based on your own needs and popular request.
I can't do any testing at home. The amdgpu firmware hasn't worked out
for my laptop.
I'm not sure I understand what problem you're having and how this relates to testing LedgerSMB: amdgpu sounds like a graphics card. You may want one when running a browser, but Firefox worked without one for a loooong time. So this shouldn't be a problem for LedgerSMB. Neither should it be for the server side.