Currently most places operate in a grey area or require a simple permit (there are some toxic towns that we'd steer clear of).

Simple- exellent relationships with reliable and honest contractors.

When I managed properties the owner would regularly call me in his office.... why did we spend $1/sqft to paint this apartment, why did we replace a fridge, what does it cost $2500 to do a simple make-ready on a 1 bedroom luxury apartment?

I think the best you can do is add a few accent walls in the bedrooms and make it less neutral. we paint everything white in our new builds and very neutral as well as do higher end simple painted accent walls and trim walls to help make the house a little different. if you really want to sell I'd get a interior designer / stager and spend 5k with a way more experience agent who sells new builds. we are licensed if you can't find someone. the way I market is very different. 

I currently own & operate two STR‘S in SoCal USA, both fee-simple, no leasehold, so I’m new to this concept. 

Normally 3-4 bed units are 1000-1400 square feet on average hence if you find a property where the average square footage per unit is 1300 but its listed as 2 beds each, there is a high liklihood you can find additional bedrooms increasing value with some simple carpentry work.

One simple fix for that would be to do a "partial 1031 exchange". 

As someone who is a former dedicated diy person and who once swore that everything can be found for free on bigger pockets or by reading books I am literally forever changed.

That is a very simple number. 

Luckily it is fee simple and I would need to read up on the HOA by-laws.