I once gave 2.5 months notice and it was perfectly fine.
I did this because I knew I was moving overseas and frankly, I knew it would be impossible for that not to slip out. I also knew there was essentially no chance they would ask me to leave early as they were hard up for trained people at the time. There was no extra work, no extra pressure, no negative treatment. The only thing that really changed was I didn't attend a few trainings (yes!!) and I wasn't assigned new professors to work with. (Job specific thing, but basically I probably would have been given a bit more workload had I been staying.)
I felt really good about how I left and I know it was appreciated. I think you have to judge the organization, but if they are as "nice" as you say they are, perhaps giving them a long notice period will help, especially since it sounds like you may want to go back in some capacity at some point. It will be fairly obvious that you didn't just come to this decision two weeks before you quit, so I can see why they might resent the short notice, especially for a higher level position which tends to take longer to fill.
Also, in general for difficult conversations, I come up with 3-4 lines, and repeat them as necessary. It doesn't matter if it is awkward.
"[Thanks and small talk]. I really need a change, so June 1 will be my last day."
Begging and pleading
"My heart is no longer in this work, even though I love this organization, so it really needs to be this way."
Hand wringing and desperate offers
"I want to pursue my passions while I still have my health and plenty of time. I'm sorry, but I'll be leaving after May."
Throwing money at you and crying
"I'm sorry, but I really need a change, so this is my decision."
"that's very generous, but my heart isn't here so I'm going to be leaving."
"Thank you for the kind words, but it's time for me to pursue my passions."
Take your three lines, and cling to them, and deploy them as often as needed. Don't get drawn in to details. If the take the passion thing and ask what your passions are because they will find a way to better integrate them into a revised role, you just repeat a different line. "Thank you, but my heart is no longer here and Paris awaits." The more details you give, the more you give them to try to argue against or negotiate over.