I respect buffets prudent style and extensive due diligence.

look at other STR data in the area and do similar style to do your best to recreate similar results 

Tan/brown neutral , and yes a little darker to hide stains....and not a Berber loop style, those look nice but are prone to damage...

I would also prepare to sell it on the used marketplace but if you absolutely don’t want to go that route then I would recommend getting it included in the purchase upfront (but still use it as a bargaining chip first).3) marketing: the best target audience is going to be a savvy house-hacker for a property this style.  

Is there an lower cost alternative to granite / quartz kitchen countertops that is still suitable for an STR?   

Also, there are more things to consider than max ROI.Most people start by picking a particular type/style of property that they like for some reason (usually a gut decision).

After visiting the property and assessing its suitability for an ADU, I negotiated directly with the owner, emphasizing my plan to enhance the property’s value.

Of those how many are 2 bed + home office or 3 beds, basically  suitable for a household earning $150K? 

Its also classic and never goes out of style.

I have helped finance some of these style loans.