What to gem at TH10

I am just about to ug to TH10. I have 6000 gems from gift cards. I want to get the best possible start and use them to help me survive the early stages of a new TH. I will have 9m E & G and 100kDE when I land. How best can I spend all this loot and gems? I have 5 builders, heroes are at 20, defences and troops are maxed at TH9 levels and I have legos plus core lavas. I am not looking for war tactics (like TH9.5) in the spend as I will go on to develop the base to max on one far off day.......

Lab to Lv8, Barch troops to Lv7.
Should suffice^^
Spend the rest on barrack boosting, if not on army camps.

anything but walls n instant finishes :)) worst investments

You should boost but if you want to gem to that on lab/spells/infernos