I am still finishing up my last affinity to L5 GH.  L5 is giving me +30 ACC.  On its own that is not making a huge difference in my performance.  L6 will give me +40 which is equivalent to an artifact set, so I expect some more help from that.  It will take 300 medals to get a L6 bonus, winning 75 battles.  My ability to keep doing that in the current state of the arena, remaining in G4 battling low power teams is an open question.

My roster is pretty much divided across all affinities and champ types, so at the moment I am not focussed on any one champ to level up.  I keep getting new ones all the time so I like having the stats laying in wait for any new champ coming along.

I have not yet seen the next GH milestone to achieve after this one, so that might steer me off on another tangent.

You need to get two to Level 6, and then after that you need to get one to Level 10.

Definitely pick 2 to focus on to get to 6, and then one to go all in on from here on out.  Otherwise you'll hit another massive wall when you need thousands of gold medals to get to 10.