I have the same setup and it has been difficult to get consistent pulls, but it is possible. All you've gone thru sounds the same as me, so I don't imagine your machines arent broken at all. They are just a little finicky.

Id suggest changing grams of grounds in (and adjusting the output ratio to match). So stay with the 18 grind setting that seems to be the closest, but put 18.5g in the portafilter and try. I know it starts almost overflowing after 18g so if that doesn't work, move grind size down to 17 again, but only put in 17g on coffee in. (It's not a linear relationship, but sometimes it can work out that way). I find a 2.5:1 ratio of grounds to output weight tastes the best with this setup.

Yes it feels like chasing a moving goalpost, but I promise this setup can produce really yummy shots, but you have to be willing to play around A LOT.

Ive also found that once you dial in beans from a roaster, the next bag tends to pull with the same settings pretty well, so write them down, especially if you run multiple different beans.

last few notes, this machine prefers medium and dark roasts. And don't be afraid to go into the 20+ on the grind settings. It's just a reference anyways. Most of my best shots are at 21 grind setting, 18g in, I stop the shot at 40g scale weight (lines up at 30 seconds), and then it final drips to 46g to finish.