I think we just disagree on real estate. I see it as the kind of thing you NEVER sell unless you absolutely have to. It's literally generational wealth (and should never be divided like OP's grandparents did, splitting single plots like that is fucking dumb and breeds resentment and competition). I also wouldn't recommend selling the house they're in now: I think they should hire a property management company and rent it out, because the extra income from that will help them get a bigger mortgage more than having a big downpayment will, and you can keep profiting from that house for a long long time.
I just see commercial real estate as being a big risk for owners like this. They only have one building and certain facts about the building (Historic area, college town, trending bad relationship with city) are usually signs that it is time to give it up and take your money somewhere else. Considering his side of the country doesn't lack in housing compared to the west coast selling the home they live in just simplifies the amount of property management they have to do. Unless OP wants to remodel and rent the house its just extra work with less squeeze.
Moving up to a bigger house isn't wrong, but the reason price per sqft goes down over 3000 is because NOBODY needs 3000 sqft to live in. That's an insane amount of square footage for interior space even if you have a bunch of kids. Location matters way more than size. Check justice map to find the location with the best demographics and you'll see that those houses have the best valuations and the most solid price histories, too. More resistant to downturns.
I disagree on the "nobody needs" argument. Nobody NEEDS to live in 800 square foot clap trap apartments, they just get stuck there. Big houses are in better shape, more modern in some cases and have less nonsense with neighbors. It can also quiet a wife who wants a new house for a while as the size of the space disarms any questions about bigger.