We do exactly this (Oct to April), both have a dtv visa.
I wouldnt rely on the border bounce system anymore, immigration seem to be getting ever more fussy on these continuous entries, they appear to be coming to an end.
It won't be imagration who ask, you have a visa. So they won't care. You could extend that time by a second 180 days. If you were to overstay in any way, I expect that will enable them to void your visa.
It'll be the airline who will ask, as you are they are responsibe for your return,,, if you don't have the correct paper work. But you do.
So when the check in airline desk ask for a return flight, just hand over your visa. This is an agreement with Thailand to enter. At that point the airline will continue. Ive already had exactly this twice.
I don't think it's actually a requirement, although I'd say it probably should be? As I'm not aware of any tourist travel insurance which would come close to covering that amount of time.
To the op, I'd certainly want a very large stash put away to cover potential medical costs,, could be hundreds and hundreds of thousends?