The title says it all...
How are you planning on starting your Suebi campaign?
I'm thinking that invading Boii first with a good offensive army is the best opening move. Capture the whole province and gain as much gold from looting as you can.
Since it's a Celtic culture you will have lots of resistance so maybe a province edict of changing their culture would be good. Also enslave as many as possible.
After this there are two wars that you could get involved with:
My plan is to take these one at a time. Wait for a moment when either both sides are weakened or for a nice undefended settlement then strike.
Also if things are tough I might simply make friends with the Cherusci and invade the Rugii since I want to have the whole Suebi province and that could get me an early trade partner and friend.
The whole situation is kinda complicated but hopefully you can limit yourself to one enemy at a time early game.