Can I Replace This Junction Box with a Subpanel?
A new subpanel here is legit.
As always, &quot;Go BIG&quot;.  All reasonably sized subpanels currently on the market are 14.25&quot; wide, intended to fit between two joists. Nobody ever lamented &quot;Why on earth did I buy so many breaker spaces?&quot; but they sure complain about the other thing.
It will need a 4-wire feeder. It looks like there are currently 15 wires in a 1&quot; metal conduit (four MWBCs of Hot-Hot-Neutral each, plus a #8 H-H-N  for the 50A circuit).  If that is non-flex, that's your ground wire and you only need 3 more wires.
That's a codevio on 2 counts: first the MWBCs are not distinctly identified which hots are with which neutral... and second, 5 circuits in 1 conduit without a 310.15(B)(3)(a) derate - the most egregious being the #8 running at 50A (which only works if it's the ONLY circuit in the conduit; the derate takes it down to 27.5A).
So all those wires must come out of that conduit and be replaced SOLELY with one set of conductors (H-H-N or H-H-N-G if the conduit isn't metal).  The existing #8Cu is good for 50A.  I would fit the biggest wire  you can fit in the conduit - 1&quot; conduit will support #4Al for 65A (breaker at 70A) or #4Cu (85A breaker at 90A).
The eight branch circuits need to be brought into the new sub with appropriate cable clamps.  They need to come in at least 1/4&quot; of sheath, then 6&quot; of wire length.  After that they can be extended by wire nuts.
50A is plenty to power the 8 circuits given that there will only be 4 circuits on each 50A leg, and they won't all be maxed at the same time.  Just the same,  think about maxing the feeder.