BATTLE PARTY SIZE

This reminds me of Wizardry: Tales of the Forsaken Land. You have six battle characters, but most of the time you will use something called allied action which usually requires two or more characters to coordinate their actions, so you end up issuing less than six commands. Issuing an allied action command however, usually toke twice or more the time it takes to issue a single action command and the allied actions were ridiculously powerful, often to the point where they overshadowed the skill of the participants. The Suikoden series also features attacks that uses two or more characters, but underutilized it by requiring specific combination of characters, making it something of an afterthought rather than a major tactical element.

If you have a party of six, I think a system in-between those two e.g. not overpowered and quick to use, but still has a great tactical value, is the way to go. That doesn't seem to be what you want though.

You have a challenge if you want to allow six character simultaneously and still make the battle system a tactical challenge.

Give characters a too uniform role and it becomes obvious what the best action for them to take is.

"My fighter is good at inflicting physical damage and bad at everything else, should I have him attack or should I let him do something he's bad at instead?"

"Most of the enemies are weak against Fire, what spell should my black mage use?"

"I have a wounded character and a healer who pretty much can only heal, what should my healer do?"

One the other hand, the more flexible they are, the bigger the chance their skills overlap. With a lot of active characters, it's likely you end up with too many actions to spare. For example, if there are four character who can inflict damage and you need someone to throw a cure paralysis item on the healer, having one of the damage dealers do it will cost almost nothing since you still have three characters who can continue to inflict damage.

I am curious how you intend to dot his. I'm sure there's solutions, but it will take some thought.