If you own cash, then you will probably have to get a loan to buy out the other person (most people don't have hundreds of thousands of dollars sitting around to buy someone out). 

I installed cabinets, painted it, and made it back into a residential rental unit.

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Likely you will need a few properties worth a few hundred thousand minimum -  how much out of pocket? 

I manage hundreds of rentals, thousands of renters.

You'll likely pull a few hundred extra dollars and will will get shorter leases that could work better for your traveling.

The problem is so far almost every fixer I've sold has sold for literally hundreds of thousands of dollars more than if I were to sell to a flipper.

That's because the city won't calculate the actual amount till a structure is built, aka it's not just a plot of land anymore.This will obvious cause your property tax bill to go up and also your mortgage escrow account as well (sometimes these things can make your mortgage shoot up a few hundred a month). 

We've helped hundreds of investors in the Metro Detroit area and are the only ones analyzing Detroit's 183+ Neighborhoods to ID the best ones to invest in. 

Not to rely on speculation, but it should get better over time (hopefully) not worse.If the marginal increase between the two options is only a few hundred bucks and that isn't worth a few hundred bucks to you, then maybe not worth it.