Good catch on the bubble! I also check at every stop, same with the motorcycle.
12K+ on my Marathons at the moment. One has a veerrrry slow leak but nothing to write home about. So far, they have been fine. I "will" take a closer inspection of the inside sidewalls before we go out next. I had a rear tire delamination on my K1200LT while up in Oregon 4-5 years go. Not fun. It was completely a manufacturing defect. Being in the transport business, safety and inspection are always primary. That includes the tow rig tires and trailer lash-up as well. Take care out there this summer!