very difficult to find a solution to the school problem rumbletum and I have more choice here than you would have in Wiesbaden/Mainz
I looked at the internationals first. One has nice facilities, one has bog standard facilities, the staff/heads very approachable, all very pleasant. As you say, very expensive for what you get. I felt I would have a nicer environment there for dd and one I personally would feel happier dealing with but I would not be getting value for money in terms of education which rankled and it meant a considerable commute either way or we'd have to move to be right next door. They do hardly any German so it's really just an English language education, very expensive and the equivalent of perhaps an ordinary state school in the UK with high fluctuation due to the mobility of families/staff. Difficult to justify the cost with 3dc
We have state bilinguals here which is what I went for in the end. I am dissatisfied with a lot but I am VERY MUCH happier with the style of teaching because it is more geared to the BNC and the English speaking staff have been trained overseas. There are families from many different English and non-English speaking countries in addition to the German families and I much prefer the whole ambience, the interaction of the dc, the creativity. I have another huge ongoing drama with it right now so no idea if I took her out of this one what we could do really.
We also have various private bilingual schools cropping up all over town and they are expensive too but less so. However they don't have normal facilities, no grounds or anything and I didn't feel attracted to any of them.
I can tell you I went half out of my mind with the whole thing. Having experienced this school after the last one, I would not send dd to another standard local German school personally. She needs the fun aspect and the creativity, project work, interesting art, good play grounds, big sunny rooms and pleasant socialising etc which the other school totally lacked.
In your shoes I would investigate every alternative to those schools that you have and only take a local German school (church or otherwise) if you have no other realistic choice.
Amongst the normal German schools there are quite a few exceptions. Your dh might know about that kind of thing. Here we have Waldschulen which are placed on the outskirts fo the forest and they seem popular, there are schools with additional focus on sport or music, there are the bilinguals and then if you like their philosophy - Waldorf and Steiner.
Don't want to be totally pessimistic and drag you down if you are making the move anyway but I have to say I have found the schooling stress factor number 1 over here. Make it as easy on yourself and your kids as you can afford to do.