Lower levels give more bonuses for your medals as the price essentially doubles at level 4 and 7. Is 10 ACC from level 7 more important than 9 different level 3s? I don't think so. But progress missions will ask you to get 2 bonuses to level 4, 6 and then eventually 10.

So, choose 2 bonuses you like the most to max them. Level others as you see fit depending on champs you use.

My choice is blue ACC and C.DMG. They will be at 6 soon if free win arena stays. The rest C.DMG and ACC are at 2-3. The rest are at 1-2. May be I should switch from C.DMG to second ACC, not sure.

I think you should take into account that the current state of arena will not remain in the long term, as per Valkyrie's latest update - although you may be winning gold medals now, it's far from guaranteed that you'll keep winning them as easily in the months to come. So if you have enough spare bronze medals that you can get some bronze unlocks without having to use any silver or gold, go for it and same with silver medals for silver bonuses. But I'd make sure to keep your gold medals for the level 7 and up bonuses, unless you're confident that you can remain in gold for the long haul. 

I have used my bronze medals.  Have some silver but not enough to upgrade from 3 to 4.  Will keep status quo until I hit the mission for lvl 4. 

Good suggestions. 

This is my Great Hall.

- I finished ACC.

- I am doing RESIST.

I explained all of this in a Great Hall Guide.