What design will minimize slab cracks?

Fiber additives do reduce shrinkage cracking, that is the purpose for them.  They don't, however, add any strength -- a common misconception.

A stiffer mix will be less prone to cracking.

The best thing is to make the cracks where you want them.  Typically that is done by sawcutting.  To be effective, sawcutting has to be done to 1/4 of the thickness of the slab.  Typically it is done the day after the slab was poured, with a diamond saw.  Concrete cuts fairly easily on that next day and has not yet shrunk much.  For your application, making sawcuts so they fall under the interior walls would put the cracks where they will do no harm.  I assume you will be installing tubing room by room.  With your sawcuts under the walls, they'll be in places where there will be no tubing.

Careful planning and measuring is essential.

Compact your gravel bed

use a dry mix, 4" slump.  if your concrete runs from one side too the other your concrete will be junk.  Water evaperating causes it to shrink(crack).

Use 25psi exp foam under with a poly vb . Then cover after finnishing to slow evaperation.