Cool, a video . . . . .it really helps a lot.
OK,
Yes, You can swap out that 50A C.B for a 20A
Yes, you can use that same #8 wire.
BUT
You are gonna have a hard time getting that wire under the set screw on the 20A breaker AND likewise under the set screw on the 20A outlet. Even the manufacturer would frown on it.
SO
You will need to pigtail splice a 20A(12ga) piece of wire on each end of the #8 run.
The larger Blue wire nuts will need to be used.
Don't forget the GFCI protection!!!!!!!
I don't have audio at my dumb server terminal, but I think I know where you want the outlet box mounted.
I would seperate the conduit at that union about 8' high. Pull the conduit off the wall and slide it off the #8 cable. Cut your conduit, deburr the sharp edges and slide it back onto the cable.
I would recommend a larger 4" square outdoor box just for the pigtail connections and mount your outlet with it's box below/next to that.
Don't sweat the NG line.
'da Kid