It still seems like it could be worthwhile in the 2-3 year timeline if you were diligent about setting up good systems. I.e. you could set up a separate savings account that you put your yearly (monthly/weekly?) donation into and then donate every 2-3 years.
For me, at least, I think this would do a lot to decrease the cognitive load and temptation to spend. I don’t do this currently but am thinking about it. If you can truly think of the money as already donated when in the separate account (maybe a wealth front cash account?) this would solve a lot of the problems. I do think I would end up donating significantly less when bunching if I didn’t keep the money separate.
It seems to depend a lot on what it means for someone to no longer be involved in the EA movement. The relevant alternative in my mind isn’t donating nothing.
Speaking for myself I can certainly imagine not being involved in the EA movement in 2-3 years. It’s a lot harder to imagine myself raiding a dedicated bank account I had set aside for donations. That doesn’t mean it’s not possible, but if use the population estimates of not being in the EA movement in 2-3 years of 25-30% (which seems reasonable) as my own risk, I’d estimate the risk of raiding a dedicated bank account for charitable donations as a fraction of that—maybe 5%. In that case it could be worth it. Maybe I’ll think a little harder about that point if I decide to do it.
I 100% agree with the principal behind your post. The future, including your future identity, is so uncertain that for almost everyone the best time to donate and form altruistic habits is right now. I would only “bunch” if I could do so in a way that allows keeping those good habits.