Fighter / Mage Dual Class, for more of a mage character

So I want to play a mage for a full 1-2 playthrough but love the proficiencies you get from fighter for a mage. The real question is what level to switch over. Here's what I want from the build

ability to use bows / staves efficiently through the game, nothing crazy just somewhat effective
wear elven chain
not fall way behind in spell progression in difficult encounters
not switch so early that I hit the cap way early in BG2

3 is kinda the easy, hey im playing a mage i don't care about fighter really choice, But then I thought I would probably hit the cap for mage

Also would choosing a fighter kit help at all?

If caps aren't a big deal I will probably go with 3

So my choices are
3 +1 prof
6 +1 prof
7 +1 att
9 +1 prof
12 +1 prof
13 +1 att

Appreciate any help!