If you need financial help, ask under the "Finance, Tax, and Legal" forum.

Lower taxes, no POS and lead testing. 

When we looked at what his cashflow is currently, and then considered what he would net out of a sale post tax, he decided to keep owning the properties.

He’s also interested in staying in the home if possible and deduct the rent from the balance he owes.

Be mindful of short-term capital gains tax, as profits from flips held for less than a year are taxed as ordinary income, and frequent flipping may result in self-employment taxes.While profits from flips do not qualify for 1031 exchanges (since flips are considered inventory, not investment properties), you can minimize your tax burden by deducting allowable expenses like renovation costs, loan interest, and holding expenses.If you're considering diversifying into rentals, explore opportunities to benefit from long-term tax advantages such as depreciation and lower long-term capital gains rates.

I would recommend an STR for the best tax benefits and cash flow no doubt.

I have 2 options:- finance the construction pre-selling the apt, and take my profits (expected to be around 700K pre-tax); or- finance the construction with a 6-7%/year loan (collateral = my house), keep the 10 units, and once the building is completed in 24 months, rent them for circa 180K/ year gross.

That will show what proposed tax increase you will see.

@Andrey ShostakA tax calculator would be nice.

Finally, even as the landlords cash flow increases, depreciation should keep a major portion tax deferred.