@Marti4182
There is no other option then the pulldown menu offers. Although i agree there should be some more choices, not even longer or in between, but also shorter, like a 1 month and/or 2 month option.
So you need to do it manually, like setting the "av-window" to "dates unavaiable by default" and open next 18 month manually, extending it at the end every week or so.
Or set to Av-Window to 12 months and open 6 months manually (but not sure if Av-Window will close them again next day, so better test it..)