^ Sounds like you have a good handle on it. (Great tenants BTW!) Factor in the cost of borrowing if you hold so you have a real apple to apple. At the end of the day, college costs what it costs, your assets are what they are (regardless of how they’re invested), your debts are what they are. You can take form assets to have no debt or you can add to assets (by not selling - or investing elsewhere) and add debt. The variables are market return (including costs) and stomach tolerance.
Maybe you can do a spreadsheet with some rough calculations and estimates. There are loan amortization templates available. I’m in the camp of keep the rentals and see how things roll with scholarships, loans, etc. With that much equity, you can probably get a home equity loan if you need to at a lower rate than the unsub student loans.