We tried to get info on the lender to ask directly but they wouldn't give it to us.I would have thought this would be a relatively quick process to do a credit check, check debt to income, employer verification, etc.current loan's interest rate is very low so they definitely haven't refinanced anytime recently.Additionally, having never assumed a loan before, anything I should keep an eye out for that might cause a hiccup? 

Unless you plan to sell your home soon, cash out refi it.  

For instance, for any reservations with VRBO and some 3rd party or direct book reservations, they use their own cc processing which has high fees. 

The process can be lengthy and complex.

The HELOC was the original plan, but it was going to take too long to close so we did this instead.My 403b has a value of roughly 85k and has a remaining loan balance of about 22k… about $700 per monthMy husband’s 457 has a value of roughly 180k and has a remaining loan balance of about 30k… about $850 per monthSince the HELOC is a variable rate, I’m hesitant to use a large amount from that.

Our plan is to sell or refinance within 15 months targeting a sale price of 3.5 million.This property is two blocks from the beach and has a high demand for short term rentals.

Also, define how you'll share the acquisition costs, such as the down payment, renovation, and closing costs.Payment Responsibilities: Explain how to divide and pay for regular costs like mortgage, property taxes, insurance, homeowners association fees (if applicable), and upkeep expenses.Management and Maintenance: Agree on how property maintenance, repairs, and improvements will be handled, including decision-making processes, funding for these activities, and responsibilities for performing or managing the work.Single Decision Point: For example, I've seen situations where one person agreed to replace an appliance while another strongly opposed it.

It might be worth asking them to start the eviction process as soon as possible next time you have a late paying resident.

Conventional loans are for investment properties unless you plan on living in a multi family as well.

I am planning to use conventional financing to finance my investment.