It takes years for band members to be able to play well together

Do not change band members regularly.

In fact, do not change them at all if this is possible.

Band members will have to take years to understand each other musically. So regular changing of members would not help in getting the band to play cohesively and tightly together.

The church should designate new band members to new bands, not trying to insert them in seasoned bands.

Inserting an inexperienced musician is like asking for trouble. Experienced musicians expect high quality playing and the least mistakes from new members. They tend to compare the new with the previous.

New members would be under intense pressure to please, which is usually beyond their present skills,  the older band members.

It would just be hell for both parties.