[Question] Quarterly retention

Is it possible to somehow configure retention for quarterly retention?
I want to have the following retention of my backups:
one for each day for 14 days
one for each week for 8 weeks
one for each month for 6 months
and then one all 3 months for as long as I have space left on my device.

So far I have not found out, how to setup "one all 3 months" in btrbk.

If that is not (yet) possible, would it be possible to make this a feature request to add this to the retention policy? e.g.
target_preserve 14d 8w 6m *q