What are you guys even talking about?
It's been said so many times before that nobody should ever count on getting a rank higher than, I think, Trusted. He's already ambassador so what are you saying he should "earn" now?
Getting a go for a Royal donation means you have earned a substantial degree of trust. I can't see how that's a bad thing.
My advice; if you have lots of ideas for big builds, do it. If you don't really need the heavier tools you will get tired of them regardless if how you got them.
Honestly Thags, your workload would be easier if you went for Royal. You are already Gym Leader on Pokeblock and the first one at that so you're likely to get the most challengers. Ambassador is quite a responsibility and it might be very frustrating trying to juggle Amby and Gym Leader. Do what you want but that's just my two cents