I think this a fine idea.There's a slightly easier and more efficient way to do this Tom unfortunately, I lost my photos of the last shop build.
layout of the outer form is the same but the inner one is suspended 6-8" above the floor rebar is used drilling into the existing floor
and securing the rebar to the inner form. The whole floor is one monolithic pour.It' left to set for 15 -20 minutes the inner forms are removed and
troweled smooth.