I play games that also have battlepass-style systems like the Mastery Track, my recommendation if you want to increase sales is to make it so that they do not expire if you purchase them, and allow for them to be purchased at a later date if missed.

If you want to drive engagement at certain times of year you could do this so that only on X Holidays, or perhaps the Anniversary for that Set's release, the Mastery Pass for it becomes available for purchase again.

Yes, it may require some retooling of your systems. My main recommendation would be to allow people to progress the current set's battlepass through the current limitations, however they choose one past set's pass that is ungated, completing your challenges awards you a second currency that progresses you through it, as do all your wins (or at least daily wins).

Driving sales further, you get to store up to say 3 levels of a tertiary pass' experience without having one active, beyond that no currency is stored. So you are encouraged to set one as active, whether one you've previously purchased, or one that is currently being purchased.

As it is, sure some people may be encouraged to buy a pass last minute and blast a lot of real money at it to expedite the leveling of it through to the end, but by the same token I am sure you are missing far more in sales by people that were interested, but realize they don't have time to complete it before the end and decide not to buy it, even just a couple of weeks in.