I just went an measured, about 18g of finely ground coffee fit into my 'double' espresso shot... However I could easily power through 5 of those a day.

I further fell from grace when I started packing as much as possible into a massive percolator for rocket fuel by the filled cup.

But you are right, 3 shots is probably not that much caffeine. It's somewhat hard to get good numbers on it.

Yes, and honestly you’re doing fine, because your 3 pulls of espresso is about the same as a Starbucks venti latte at 180-190mg. I think in Europe the caffeine content is even higher at something like 225mg for the same latte.

If you really, and I mean really want to have a go at the rocket fuel territory, you can do what many cafes in Istanbul now offer: Turkish coffee… at a regular coffee size. I think it takes about 150g of ground coffee per cup. That’s like pulling repeated shots of espresso till you fill a regular mug, except Turkish coffee is finer ground so has more caffeine than your average espresso per gram of coffee in.

Obviously, don’t try this if you have any medical conditions or sensitivity to caffeine, but this is what some Turks often drink on their days off.

Back 10+ years ago when I had my machine 21g baskets were all the rage, and I think I remember whispers of larger.

Yes, that used to be the case I think. The espresso world backed off of that in the last few years. Nowadays the average dose is trending lower. Possibly, half the reason is that a larger dose is more inefficient (i.e. it drops your extraction rate) and the other half is that coffee costs money and for coffee shops that operate at 5-10% margins, 14g doses (standard Italian double espresso) vs 21g (the ‘third wave’ dose up till a few years ago) does make a meaningful difference in whether the store is under water or not. I think it’s settling around 16-18g.

Operating margins, not gross margins right? Would you happen to know what's a "typical" wholesale cost (i.e. to the café) for third-wave beans?

Yes. James Hoffmann on YouTube talks more about the margins of coffee business if you'd like to learn more, that's where my knowledge comes from for the most part.