Say they sold the properties over 3 year period and could not sell the rentals together as a portfolio.Could they put the money from each sale into a DST then use the total to buy a expensive rental property?

Unless you revise your plan and ensure that your personal use is under 10% of the rental use.

2023-2024 DECIMATED my already broken finances and killed my 15 yr run being debt free with paid off rentals and a house near the beach in the Ca.

Quote from @Sophanara Khoeun: As titled, I am new to the property rental scene.

Your cousin should contact the lender to discuss his options.If he rents it out, he must report rental income and expenses (e.g., mortgage interest, taxes, repairs, depreciation) on Schedule E starting when the property is placed in service.

Do the math and see if an ADU is worth the cost and effort.https://www.biggerpockets.com/blog/rental-property-cash-flow... 

I'm interested in learning about rental property investment, looking to build a rental portfolio, exploring real estate investing to replace my income.

Research local rental demand, especially from military and university tenants, and consider balancing your portfolio with single-family and multi-family homes.

Verifying last 2 years of rental history very important!

I have been dabbling in real estate investing and we have a vacation rental property in Custer SD.