I'll figure out something, I always do, I just thought it would be nice to see what route others have taken with their own servers.
Yeah for sure. Sorry I don't have a good answer
turned their email over to Microsoft and we've had nothing but non-stop spam, phishing attacks, outages, and the constant push of "oh if you're not going to use a Microsoft product.
Just wanna share that my experience does not mirror this. I pay them $6/ user per month (which is just me, for me personally, to be fair), which gets me that hosted exchange server 365 thing. I only rarely, if ever, need to use the other office products, and I do so in my browser. In the 2ish years so far I've had no complaints. I don't require any of the features that are locked behind full-installation variants of their products - and besides that I've had no problem with spam email especially.
Im not sure I would recommend that you tell your friends to authenticate with your own Active directory instance necessarily, but ultimately at the end of the day if you're dealing with users you'll need some kind of authentication layer (imo)
If you want you can install starship, which provides a lot of the nifty prompt customization I see in other people's zsh configs. I've been using it with bash for a few months now and like it a lot.