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If you have a new igniter with the proper gap the a dirty burner is the most likely cause of delayed ignition (the bang). You to remove the burner cylinder to clean it properly. You'll need a new burner gasket to put it back together.
Vacuum it out and wire brush the burner cone if any of the holes in it are clogged
You have to remove it first. Of course if there are any defects in the cylinder then it should be replaced
You have to use a fine soft brush if it's clogged up. Usually a brass bristle brush. You certainly wouldn't use a stiff steel wire brush. Also carefully tapping the burner can dislodge the debris.
The "brillo" is on the outside of the burner. That's why you have to remove it first if you want to clean it.
You need to remove the burner to do this properly. Any loud bang is delayed ignition and not safe
Then the only other likely thing is the electrode gapping/positioning.
I would have assumed that was done during initial set up but it's worth a shot. I also wonder about fan speed since you say it now only ignites late when it's hot. But that's a control issue and there is no diagnostic for it.
I'm logging off for the night. If you have more questions I'll respond in the morning
You need a combustion test.
You might want to contact HTP tech support. (###) ###-#### ***** this point the only thing I might try is replacing the burner.
No it's not normal. Neither is the condition you describe after trying all the remedial methods.