If you add 60 days plus your visits to Thailand from mid December 2018 do you get 180 days or more? It seems that some IOs are enforcing a non-existent rule of 180 days or less. What if you add 6, 8 or 9 months over a rolling 12 -month period? There are now many recent threads about people having issues with METVs. It's almost like the IOs have instructions to scrutinise anyone with an METV. Hmmm...
As others have suggested the agent-assisted retirement route is probably the best option for you. Should health insurance be added to the retirement visa requirements at a later stage you may be grandfathered into the old scheme. If health insurance becomes a requirement for your retirement visa, the agent may be able to handle that as well. If they can't, you could either stop with the agent and switch back to an O-A visa or pay for an Elite visa at the time (if it still exists).