It's never rings and as such it is never a breathing problem. If you aren't fouling plugs with that much consumption it must be even in each cylinder; which is good as if one valve guide seal was using that much oil it would be the result of a bad guide.
Valve guide oil seals are the answer. We usually pull the plugs and go for the cylinders with the most carbon on the plugs first. This is to try and find the bad guides first before replacing all the seals. It's a 5.5hr job to do all cylinders.
There is no diagnosis, you just do it. If you come across a bad guide then its valve job time.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician