Regarding balancing teams using leftovers after putting the best girls in the helper squad, why not use a different squad page entirely?

Page 1 has only the helper squad, filled with your best girls.
Then use page 2 or onwards for your own mission usage, balanced/sorted to however you like.
Just remember to choose the correct squad page before you go into a mission.

Balanced speeds are only necessary for 3 medal clears. If you're experience grinding, then it really doesn't matter. Where you mostly should be concerned are event maps where you repeat for currency gathering. On some of those maps, completely balancing teams by power or attack is detrimental to actually clearing the map. Never mind 3 medal clears, you want efficiency, and you want to make sure you're not going to hit a cockblock on RNG.

In other words, you should really make use of the other 5 alliance slots you're given when creating compositions based on event maps.

mhm, I've organized my page like this

1 - trophy case, highest power to show off in the rankings with the best ones in the helper squad
2 - balanced, with roughly even power and speed
3 - event team
4 - 1 really strong and fast team, with the others balanced
5 - waifu team
6 - raid boss team, just a single 2* girl