I jumped at the chance to get J&J even though I'm not a essential worker or anything. I think the disconnect between our intuitions is here:

People who can easily continue to guard against significant COVID risks for several weeks without much downside other than quality of life should wait several weeks for Pfizer or Moderna.

As was discussed a bunch in my post on lockdown, the quality of life & mental health impact can be really massive. A marginal month may not seem like a lot if you are really just doing totally fine in lockdown and don't have anything in particular you want to do, but if you are actively suffering all the time, another month feels like forever. Also, for me, waiting a month would mean that I would see my niece for the first time at 3 months old rather than 2 months, which is quite a big difference at that age.

Furthermore, I'm young and healthy, so getting a slightly less effective vaccine probably just shouldn't matter to me that much, when my risk was already so low to begin with. And I wouldn't be surprised if people who get J&J now can get moar vaccinated later (either a J&J booster shot or stacking an mRNA vaccine on top of J&J), so I'm not convinced that the choice I make right now matters that much.

Also I'm pretty skeptical that J&J provides 100% protection at any point, and your source did very little to convince me. 0 and 1 are not probabilities?

For the sake of argument, let's assume that the effectiveness of j&j and moderna/pfizer are 66% vs 95%, respectively, and this effectiveness comes in at the same time. Suppose you keep yourself to a constant risk of x per week, without factoring in vaccine protection, based on the case rates and variants in your area. If n is the number of weeks between getting the j&j vaccine vs the moderna/pfizer vaccine, and m is the number of weeks between getting the moderna/pfizer vaccine and the end of the pandemic, then you have the risk of (1-(x*.33)^(n+m)) if you get the j&j vaccine, and the risk of (1-x)^n * (1-(x*.04))^m if you get the moderna vaccine.

If I was offered the j&j vaccine tomorrow, vs the moderna vaccine I have scheduled in 3 weeks, and we assume the pandemic will be over in the first world by september 1st (20 weeks out), and my weekly (non-vaccinated risk budget) is 200 microcovids, then I would be looking at 0.132% chance of covid with j&j vs. 0.093% chance of covid with moderna/pfizer. So, it's worth it for me to wait it out. But, if you think the pandemic will be over in half the time, then it's 0.066% vs 0.085%, and so you should take the j&j vaccine.

I wound up with Pfizer, but I actually would have preferred to get J&J, due to the more established vaccine tech with less risk of allergic reaction.  They work similarly enough (put *NA into your cells, you build the spike protein yourself, and then react to it) that it's hard for me to believe that much of the apparent difference in effectiveness is real.  J&J scored worse, but on a harder test including the newer strains.  So I imagine J&J is comparable to the first shot of Pfizer/Moderna, and although the second shot does make a real difference, the obvious solution is to just get a second shot of J&J.  Ideally you would wait until supply caught up with or exceeded demand, and ideally the places administering it would stop caring at that point whether you already had a shot but might charge you for it.  In the (likely) worst case where they don't allow it if you already had a shot, you could lie.

A lot depends on how much personal control you have of when and what kind of vaccine you get.  If you knew for certain you could wait 3 weeks and get your preferred vaccine, that's probably better than taking J&J today.  But if there's a fair chance that you WON'T be able to - either you'll have to wait much longer or take J&J anyway, you're probably better off just taking it now.

The driving factor is just how much COVID-19 sucks, and the cost of getting it during that voluntary gap.  If you're truly comfortable and truly locked down, then waiting longer is more reasonable, and it also lets people who need it more than you get it sooner.  In that case, delays of up to a few months may be justified.  If you're only mostly locked down (as I am - I still go out briefly a few times a week for things that can't easily be delivered), then delay is riskier and you should prioritize any vaccine, delaying no more than a week or two.