Properly diagnose coolant leak after head gasket repair?

I bought a BMW E46 330Ci (M54 3.0L Engine) that was not properly maintained. It was leaking coolant and sputtering white smoke and water from the engine (didn’t sound smooth). Coolant pipes were also broken.

I pulled the engine out, and replaced all the seals and gaskets and coolant pipes. I sent the engine head to a machine shop which pressure tested the intake and exhaust ports and said they were okay, and then skimmed the head.

I checked the block with a straight edge and feeler for any warping and it was within tolerances.

I put the engine back together and then back in the car. The engine feels really smooth and powerful, no more sputtering from the exhaust. The first few days, there was a strong odor (my guess is previous coolant in the exhaust system). I’ve driven the car for about 2 weeks now, there is still a noticeable odour, although nowhere near as much as before. The coolant level has dropped from max to min (I topped up once as well).

Torquing the head bolts were very difficult on an engine stand (didn’t have a very long breaker bar either), and a few bolts, the 2nd torque angle I might have applied 70-80 degrees instead of exactly 90 degrees.

Here is a photo of the block after cleaning it, just before putting the head back on:

My next action is to do a leak down test. If I see bubbles in the cooling system, will this confirm that my head repair was not done correctly? Where could I have gone wrong in the repair?

Is there any other tests I can do before having to pull the head off again?

Edit: Photos of the spark plug (cylinder 1 & 3, both look the same) and the expansion tank cap. Would a half broken expansion tank cause coolant loss? There is no visible white powder stains near or under the expansion tank.