Hi Bradley, Welcome to BP. I'm in a similar situation to you but without kids, and I'd LOVE to have your background in architecture and design. I think it'd be worth paying a tax advisor for an hour or two of time to get their take on the implications of either road, but here's my take on it.
Interest rates are still very low and will eventually go up (my parents' first mortgage was at 18%). Also, with a mortgage, you're paying it back with inflated dollars meaning you're really paying less. Jason Hartman talks about this a lot. Here's one of many articles/podcasts of his that discusses this.
If you know you want to end up in REI, why not start now? As with investing in the stock market, I think most investors look back and always say they wished they would have started sooner. I think having the opportunity to get into it while still in your 20's awesome.
My wife and I spent the last three years remodeling our house as well. We don't have kids, so we rented out two rooms and lived in a slightly separated basement. It wasn't always fun, but it was AWESOME living mortgage free. So I get that. When we do have kids, our plan is to look for a house that makes sense to build a detached accessory dwelling unit on the property. Then we could have the benefits of our own house plus some rental income. I think that would be something to consider with your background.
I also like your plan on multifamily. You could purchase a modest home for you with an FHA, as well as a moderate fourplex and have 5 living spaces locked into 30 yr mortgages at attractive interest rates. (I'm biased here, though, because we put in an offer today on a triplex for this exact reason).
Best of luck and keep us all up to speed on your thought process. There's lots of young families out there wrestling through these same decisions that will benefit from the discourse.