Typically, if you have a mortgage, the mortgage company will collect taxes and insurance when you pay,  so there is not really any budgeting required for this.

Is there typically an ROI?

Open houses are typically used to generate leads, not sell houses.

Are they familiar with what they should do for colors, amenities, etc?

A cash out loan would have 6+ months typically and if it is a traditional loan it would be 12 months.A DSCR loan will not work for what you are trying to do.

It does share similarities with annuities, but the structure is designed for a specific audience:Property Owners: Typically those who already own income-producing properties but need liquidity for renovations, debt repayment, or other opportunities.Investors: People seeking exposure to real estate cash flow without the complexities of direct ownership, especially those already familiar with stocks or REITs.Again, the idea of the model isn’t for everyone.

In any contract you don't typically need a clause saying what you CAN do, its assumed you can do anything legally allowed as an owner including sell.

typically with seller finance there would be some period of monthly payments and then a balloon where you pay off the balance in full. 

These loans are short-term as you mentioned, typically 12 months, so you'll need to refinance before that balloon comes due.

The IRS generally does not allow you to deduct expenses for a property that has not been placed into service, as deductions typically apply to operational properties actively being used to produce income.