For example, my circular saw seems to run at about 1/4 speed.

Look for evidence of nails/screws failing- little circular holes in a line.

Power tool wise obviously a circular saw is a big one... the saws all is a great go to as well.  

I was thinking of exposed, circular ductwork attached to the ceilings to save money on soffits and give the work more of a loft feel.

Prop 2 = a old stone one room schoolhouse that is actually 2 stories, 3 bdrm, approx 1,500 sq ft, full new roof, structure solidified, mostly cleaned up inside but also a full gut from floor, walls, ceiling, some parts only framed out, a few elegant details in place like the original slate chalkboard & a circular staircase (surely not original ofc, but nice).

I'd turn to CBRE cap rate surveys or Marcus Milichap's MFR circular for 2022.

I needed to clarify @Mindy Jensen, let me try it again; would the cost to skim coat and texture a circular pattern like this, cost the same as removing a popcorn ceiling, about $3,000 to $4,000 depending on size of house? 

Correct it's a circular calculation- so the gain will go into your overall income determining bracket and the rate applies proportionately 

Changing access to the basement to a circular stair or something that takes less room would enable me to join kitchen with dining and make it more liveable for us but more valuable for resale as well.

Did you cut the cabinets with a circular saw?