can crew use it to travel in premium cabins on a regular basis and if they chose to do so, pay slightly more to guarantee a firm seat?
Without getting too specific:
After 6 months (I think) you can buy an unlimited number of basic economy standbys, or more expensive enhanced standbys which improve your chance of a seat in a premium cabin.
Each year the individual also has an entitlement to buy a (very) limited number of bookable tickets which have enhanced boarding/premium cabin priority...however even though they are sometimes called "firms" you are still vulnerable to being offloaded in favour of commercial pax.