Where to focus your energy, time and money.

In my opinion it heavily depends at which state your character is. For example a new level character should be leveled up as fast as possible, then you should try to get his gear up to a point where it is really hard for you to find any more and in the meantime you could start collecting shards.
On the other hand, if you got a maybe 5-6* Character it is more useful to go for gear, especially the higher gear levels give you huge boosts.

I think its hard to say what is best to spend into in generell, just take a character and spend all your time on finding stars and gear for him and if you reach some kind of "wall" then go for the next one.

Should go, Gear, mods, character level, then Stars in almost every case. Star level adds health alone and especially at 6* and 7* this health boost is minuscule. Gear however completely changes a toon almost as much as mods do. Same goes for character level. Character level is useful for the toons damage and defenses. However the gear should be the primary focus