JMK with CAT is a very strong team m, probably the strongest in the game. There are counters to every team though, having a single GL squad is no longer the ‘easy button’ it was in the Arena meta days of old.
I don’t have JMK, but would consider him as a really strong first or likely second GL (After JML). Lots of Jedi team comps for GAC, TW and LSTB with those 2 GL.
It’s a long grind for one, but JMK does include some strong toons in the requirements (GG as noted already, GK to R8 for fleet, Jango for BH and Mandalorian teams, and Cad Bane for fleet).
Most people focus on GLs, but another option is to develop 4 strong synergy teams instead. That is the trade-off in time/gear. If you already have 2-3 strong toons, you can fill out more than 4 teams with the same investment.