Charmion, it won't work for your concert choir, but
this is my chorus's secret for attracting audiences that are SRO.

Perform with groups that have young children in them.
All their families and friends will buy tickets and show up.

Our chorus performed in a show with:
a troupe of Irish step-dancing students
a children's chorus

and the hall was absolutely packed with people.
This was a big deal for the children's chorus,
which had been inactive from the time of the pandemic,
so no children's chorus for some three years.