We’re sick of the heat and want to live somewhere with 4 seasons!

With one good in season rental week it pretty much pays for itself to have 3% fee all year.  

You can also consider buying materials during off-season sales.Tight Project Management: Keep projects on schedule to avoid costly delays.

Here's some seasoned arbitragers that can help you decide if arbitrage is the right fit.... 

However, now that we’re heading into fall, we can see that 2024’s summer selling season was slower than 2023, with closed sales in Austin down around 16% compared to last year.

I am exploring renting my STR to MTR option when the summer travel season dwindles.

Having a good broker that can tailor the loan to your specific situation is better because they will weed out the lenders that may not go down to your loan amount or may require seasoning of funds or may not allow short term rentals etc...

If the condo can be used for MTRs, can I change rent across rentals in a year depending on seasonality and target segment?

When you have value to offer right away you make better connections with buyers and you need help finding a buyer you can partner on your first couple of deals with a seasoned wholesaler. 

In some markets, seasonality increases rents in the summer vs. winter, but this typically runs ~5-10%.