Hi Milibear
I am in Bavaria and am a Hausfrau (haha) so didn't reply earlier - my kids are 6, 4 and 6 months and I'm married to a German. Where we live in the Bavarian countryside there are not many working mums in a full time career sense (I do a bit of English teaching in the eves and used to do one day a week before DC3, but that barely counts), but I think it is very different in Frankfurt so won't address that side of things!
Maybe you could offer speech therapy in English as you will be based in a city with lots of ex-pats about - I have seen posts on Toy Town asking for recommendations for that so there may be a market (but I only look at Toy Town Munich).
You can ensure your dd is fully bilingual but you absolutely must always, always, always speak to her in English, including in front of non English speaking people (some mums say they feel rude, but tbh why does anyone else need to know what you are saying to your child, plus most people understand English to some degree anyway). Also as she gets older (from 2 I'd say, depending how she develops speech wise) insist she replies in English if she wants a response from you - the same way you wouldn't give her a snack until she says please, it can be perfectly natural to say "In English?" instead/ as well as "what do you say?... please..." :)
My dd was 19 months when we moved here and has had no problems at all with her German whatsoever, the KiGa teacher who took over her group after she'd been there a year said she couldn't tell she was bilingual at all until she heard her speak to me, and was amazed at how she switches languages. She started school in Sept and is currently doing very well with no suggestion she will need extra help at any point, though you never know of course. Her English is also totally native speaker though - if you met her in the UK you would never guess she also spoke German.
DS1 who is 4 was born here and does drop odd bits of German into his English - the one annoying me atm is "bei" - "Last week I was bei my friend Paul..." when he means with someone or at their house... but it is a minor thing which will hopefully iron out soon! I have been told that his German is ahead of some of the boys he is friends with who are his age who only speak German (well and Bavarian dialect - that could be the problem!), although he also drops the odd English word in - but then so do some Germans!
We speak English at home even though DH is German, we made a decision to keep it as our family language, and we do have English TV :) It is really important to me my kids speak proper English as well as German, as I too heard too many mums who say their kids don't/ won't speak English, and met a couple of American/ German families in which the kids of 6, 7, 8 years old speak really strange English - American accent obviously, but German grammar/ word order and some German vocab! This seems to me a terrible shame when one parent is a native speaker!
I taught DD to read in English, I read bedtime stories to all the kids in English (DH reads in German but only once a week, we possibly need more of a balance) - it is important to me they are bi-literate not only bi-lingual, and I think as thy get older they need to read equally happily in English to keep expanding their passive vocabulary etc. otherwise they are not truely bi-lingual, but German speakers who speak fluent English - which is a great thing in itself, but there is no reason a child like yours should not be truly bi-lingual with no concious effort on her part, just a bit on yours :)
Good luck with your plans!