If you rent longer than 30 days, you’ll skip a large portion of the taxes and fees levied on STRs. 

Everyone recommends Quickbooks, yet it is not a one-size-fits-all solution esp. when its annual price can be $1000+.

It sounds as if you're not happy with the asset, the size, the unit mix, and its performance.You could wait until rates drop, and find someone willing to pay a higher price than now.

You may have a grasp on this aspect, but there are items to consider that can play a big factor when choosing your land such as topography, existing zoning & platted lot status, minimum/maximum requirements such as parking, detention, size of building, setbacks.  

It will depend on the size of the house, the scope of work, and the quality of materials used.

Time will tell how many a "significant portion" is exactly, as they seeks to turn around the struggling business.

I personally know several families with spouses and children that rent out a portion of their home (finished basement/private entrance room, etc.) that offset their mortgage substantially.

What is your experience / track record with this type and size of investment? 

We are sandwiched between well known hot spots and its an ideal location for someone who wants to explore the beach life or any of the down-town areas nearby (Los Angeles, Long Beach, Redondo, Manhattan, Southbay Etc), new build with modern amenities, washer/dryer in unit, full sized fridge, dishwasher, professional range cooktop, queen bedroom, full standing shower, large kitchen, full sized couch in the living room, private front patio. 

Not only is it not too much of a commute for most renters, it is actually a preference by a sizeable portion of the rental pool.