How about as soon as we get +4 shared war with Bismarck and +1 more in about 5 turns from resources, we pay him to Cease Fire with HRE?
That way Aachen survives without much of a scratch, and it can stand up to Arabs and France who will come later.
That lets us wipe out that tasty island city for good money.
Bismarck will be Friendly with us after the Cease Fire and +1 from resources, so he can't plot against us.
The +3 shared war will never decay, and we can bribe him to DoW HRE in the future again if we ever need +4.
He is pretty far ahead in tech, so Arabs and France probably won't pay him to immediately Dow again.
Bis will have Astro very soon, so the island isn't invincible.