Frankfurt is an outstanding race. I'd highly recommend it. Beware that it has a short cut off time - I think it's 15 hours. If it's hot the swim maybe without a wetsuit. If either of those things are a concern for you then you may need to think hard about it.
In terms of organisation it is second to none. In terms of support, it's right up there.
My only fear for you is that it may already be full.
Actually, thinking about it, the biggest issue you'll probably face will be track time. You're highly unlikely to get sole use of the track at the weekend for 3hrs, you're more likely to get a Friday am or pm slot if people are prepared to take the day off work.
I guess if you start the ball rolling now and there's no joy with Manchester, you could look towards Newport and other tracks.