I’m considering selling it and selling my primary residence to build a duplex where I’ll live in half and rent half. 

Remove that burden and houses start to look a lot more affordable.This is just more click bait.  50 year mortgages area a better headline than, plan review and permit costs will be reduced by 25%

I regularly buy 80-120 year old buildings and the biggest expense I’ve had that was not planned for was putting in a new larger hot water heater in a location where the old one wasn’t quite keeping up.

Your W2/day job income is the primary driver of your wealth for the first half of your career. 

If you plan to use the home during peak season, that’ll definitely cut into profits.

Build credibility fast with a one-page plan that shows your why, your buy box, your team-in-progress, and how a lender or partner gets paid.

If so, what do you plan to do with your rates in response? 

Housing accounts for about 18% of GDP and I am guessing half of that is rent, but the other half is sales and the related ripple effects of remodel, moving, furnishing.

That being said, does anyone have advice on things I could do for the next year and a half to two years to better prepare myself? 

That sounds like nothing on the surface - what's a half a percent, right?