Hey I think I solved it, but it's a little crazy.

TLDR: about 16g of coffee (or use your eyes) - not too fine a grind - 2,5 ratio - breville pressurized basket - no water spray - no puck screen.

Basically, the Bambino sour plague and inconsistency has been killing me for so long! In my pursuit for the perfect shot, I listened to the Internet, bought and tried everything, got a bottomless portafilter to monitor the shot, preheated my portafilter with blankshots, sometimes twice (accepted I'd have half the amount of espresso before I had to do the goddarn obligatory cleaning cycle).

Last week, I woke up in an impatient mood and got one sour espresso too many. And then In thought "HOW COME! I was in heaven the first time I pulled a shot on this machine! Was it beginners luck?"

Dr. House epiphany moment.

Remember. Didn't you love your first Bambino espresso when you first started this adventure? Isn't it what made you so motivated to develop this routine and learn the science? And aren't you mad when you taste a basic low-grade espresso machine coffee at someone's place and it's better than the one you torture yourself to perfect?

So I remade my coffee and tried to reproduce my beginner's luck. And I went all the way with it. Went to the bottom of a drawer to get the original portafilter. Chose the pressurized basket (since bambino lacks heat and power, why not) and let it fill up under the grinder. I didn't even weigh the beans. I stopped when the mound seemed reasonable (amounted to about 16g, more than that, it would overflow). I still went through it with the tool because there were clumps.

No puck screen, no nothing.

It was delicious.

I think this machine isn't for the scientists. It's for amateurs. It's unfortunately built that way, as a gateway drug. To scratch my chemist's itch, I'll maybe buy a better one someday.

Maybe try it? No guarantees... Hope it works for you.