CX also has a by-invitation only Diamond Plus tier. My understanding is that this status is awarded to people with considerable spend both in relative and absolute terms (e.g. be among the top 1% spenders among CX Diamonds, threshold of which lies much north of 100k USD annually). The mid-tier benefits are not a tier per say, rather milestones to incentivise more flying on CX/OW.

People on the CX forum are probably able to provide more in-depth knowledge about this one.