What do you do?

Somehow barb carts (with giants and bombers) still works okay despite the heavy nerf

There are no penalties for upgrading to the next builder base before max unlike with the main village where you get less loot. It can be more beneficial actually since you get more defences to play around with earlier. Personally I make sure my buildings are the level of the current builder hall -1 level (at bh7 have level 6 defences)

I went straight to bh8 without even having a capable offense. when I went up the only thing I had decently leveled was level 10 barbs, and giants and archers iirc. of course, since you're matching with people with similar amount of battle capability, it doesn't matter too much. The third pair of collector at bh8 is much worth the jump for passive loot

I would usually make sure the troops I use the most are maxed for that builder hall at least. For example I use barbs and bombers for many of my attacks so I want to max them ASAP (if you are using a variation of different attacks then maxing the support troop I think is the best option since they are versatile in different attack strategies). As for the battle machine once you got the electric hammer ability it’s alright. It’s fine for it to be a bit underleveled on the battle machine (obviously too much wouldn’t be good like having a lv5 battle machine at bh7 is definitely too weak)