You're right that is a unique situation. As much as I'd like to do the same (go on a farewell tour on their dime) if I did, I think I'd wait till the start of next year to announce. Put a quarter, and a holiday, between the trips and the announcement. Come back first week of Jan and announce would be my vote.
Too soon before and the your role and the trips will be yanked for you, even if the tickets are purchased. You'll be off the agenda, and off the trip, even if you are staying through September. Which would make a miserable last quarter to work.
I wouldn't worry about the fiscal planning, that's the kind of thing that's always there.
I mean sure, you could do plan A, go do the trips, announce two weeks after you get back...and as much as I'm in the "little notice possible" and "they don't owe you anything" camp, yeah...it's kinda cheesy that way.