The join between the group head and the boiler opens up slightly and leaks (there's little bubbles) when I am pulling the lever hard for a a shot. Typically my other hand is holding the boiler cap.

Is this normal? I've read notes about pulling too hard on a 'choked' shot and breaking the lever, I am not weak, but far from breaking the lever, and the shot isn't choked, it typically gets 10s preinfusion and about 20s solid firm pull.

Getting superb coffee so not an emergency. But being a bit OCD...

It may be time to replace the O-ring and/or tighten the two GH bolts.

Be cautious about trying to fix this by tightening the group bolts. The boiler flange is brass and the stock bolts may be a little shorter than optimal. You certainly don't want to damage those flange threads with overtightening. If you snug them up (~~ 7 ft*lb of torque on lightly lubricated threads) and it still leaks, replace that o-ring before trying to fix it by going tighter.

An easy way to train your arm about when a pull is too hard is to put the machine on a bathroom scale, with something behind the base to prevent it all from slipping backwards, then zero the scale and try some pulls on a tight shot. Anything over roughly 30 lbs on the scale is pulling harder than you need.