It all depends on what your requirement for cash flow is.

Have you looked into shifting to an STR or MTR (depending on your location)?

Where you pick will depend on your goals for cash flow and your budget to buy.

Some lenders offer creative solutions, like a cross-collateralized loan, where they use your existing equity as security without refinancing the duplex.Another idea is partnering with private lenders or exploring a second-position mortgage, depending on your lender's terms.

We rent it in between $75-125/night, depending on the time of year.

You should have a good idea of every expense you have in your primary home, then add the STR specific:  (These are rough estimate numbers, depending on various factors)STR insurance - a special insurance that covers both the property and your liability, could be $2~4k in areas without natural disasters, could be more than $10k in some other areas.cleaning fee - approx. $50 per bedroom, so a 4-bedroom house will cost you about $150~200consumables - bathroom supplies, kitchen supplies, beverage supplies: $20~30 per week, depending on how much you providewear and tear - towels, linen, kitchen utensils, some furniture: $1000~2000 per yearRepairs/replacement for things that guests break or steal:  $1000~2000 per yearAlso, if your personal use is less than 14 days a year, the entire property's expense is deductible against STR income, but if you use more than 14 days, then you will have to calculate the proportion of how many days it's rented out vs how many days you or your family uses. 

I think they used to go up to like 80-85% CLTV depending on credit score.

Rate will also be in the high 9s depending on PPP and credit.

It will depend on your preferences and how well you think you will tolerate having a room mate as you approach a new life with your partner.

On top of that, depending on the severity of the structural issues, you will also likely run into problems getting insurance on the property or wind up needing to purchase very expensive insurance.