I almost wonder if existing CIPA rating methodology might not be the best indicator of battery life for mirrorless cameras. On DSLRs, very little power is needed between shots due to optical pass-through, even if the camera is "on." With mirrorless, probably about the same amount of power is required as long as the camera is on and active, regardless of if you're taking a picture. I would almost expect mirrorless to behave more like a laptop or smartphone, where battery drain is tied to active display usage.
The CIPA ratings are absolutely terrible as presented.
It should be updated to either report as minutes of runtime (which is what you are actually getting) or be updated to a more modern standard entirely. Luckily the runtime math is easy (divide shots by 2 and there's your minutes) because of the 1 shot every 30 seconds procedure.
The primary challenge is simply that taking a picture with a mirrorless camera has only minor impact on the battery, running the sensor and EVF/LCD is the primary power draw on modern cameras (unless you have an onboard flash, which when active is the only other major power draw in the body.
The methodology was developed for early DSLR's, which tended to be very exposure-bound in terms of their battery life (my *istD and D50 certainly were)
But that changed a long time ago for DSLR's and was never right for mirrorless. My D300 for example was almost purely card write limited in terms of battery life, I got ~14GB per battery regardless of what I was doing. That could be a few thousand JPEGs or much fewer RAW files.
That was a 2007 era body. The only time that wasn't true as when using live view where it was minutes of runtime because it absolutely drank battery to run the LCD.