Annual is unappealing to most subscribers - but some percentage will go for it if it's available. Like 10%ish (may vary but definitely a minority). This minority will smooth your cash flow quite a bit.
Annual plan only available after 6 months of membership?
While I understand that sounds counterintuitive, if not odd, it:
a) offers an incentive to stay.
b) with that goal line in mind, the offer of annual when getting there has special meaning. As opposed to a random "you've stayed. You must be happy. Wanna go annual?"
Again. I hear what you're saying, but in a nudge-y / Hooked sorta way it might work.
I took it.