I've included a preliminary floorplan below.My goal with this ADU is flexibility in terms of renting it out either short-, medium- or long-term rental in the future for income, as well as maximizing resale value (main property is 1300 sq ft 3/2) to someone probably with an investor mindset who would want to house hack.My main concern is whether it's a good idea to have 3 en-suites at the expense of having a small living room/kitchen area.

House-hack, house-hack, house-hack.

$75k is best used for another house hack that way get a better rate and lower down payment meaning can put some of the money in rehab.

My sister is selling her house to me; however, we want to do a mortgage transfer since she currently has a 2.5% interest rate, but the bank will only pay what is owed on the current mortgage.

Enjoy the increased cashflow and look for ways to house hack so you can reduce or eliminate your housing expense which is one of the largest expenses anyone has.Imagine a lifestyle where most of your personal debt is paid off, you have reduced or eliminated your housing expense and now have the cashflow to focus on making your money work harder for you through real estate investing and maybe can free up some more time to do that now that you have reduced your living expenses and can work less.To your success!

Force you to pay  a couple thousand extra in housing expenses and then moved back there is zero chance you would do it.

They are not assigned a house.

I talked to 2 property management companies who I should rent the house out to one tenant/one household (e.g a couple or family).

Housing is a necessity so as long as you are in a decent area for rentals it will rent, but it will come down to price.

I've lived in my primary residence in Reynoldsburg, OH since 2015, house is almost paid off (just a few grand left).