Can't wait for it to come out! Literally because i'm very impatient... I hope he's still working on it and not going to abandon it.
There is already a mod with a fire truck in it. Its called 'ETK 4000 & 6000' or something similar. You can search it on the repository.
It's broken unfortunately. Lightbar isn't applicable and the steering jams up if you turn too far. But yeah, you're right, this'll be the second firetruck, but hey, better than zero
but it doesn't break, and theres a fix for the lightbar https://www.beamng.com/threads/fix-of-the-lightbar-for-the-etk-6000.51362/
There already are some German siren mods, this one is pretty good https://www.beamng.com/resources/german-siren-pack.9353/