New oven/range wiring
Yes, that is acceptable. You don't need a junction box at the service panel end. You can have a pigtail in the service panel. You also don't need another box for the oven. Just remove the existing receptacle, pigtail on some 12 gauge wire, and put in a standard outlet.

Keeping the 8/3 in place also allows for future replacement with an electric oven by swapping out the receptacle.