Maybe have the different parts come in for different rehearsals so that they can practice their parts just as their part, not as full choir. Or you could group S/A, T/B, which also works.

Thanks MJO.

As for learning by rote. This is sometimes a necessity in volunteer choirs. However, you might take five or so minutes at the beginning of your rehearsal, or at break time, to give a brief tutorial in reading music - covering very basic things like staff reading, note values, time and rhythm, and don't overlook intervals. Keep it short and simple, yet informative and enjoyable. It seems to me that this may open some doors for growth in your choir.

I've tried this in the past, but the older folk (65+) have an entitled: "I've done it this way forever... I don't need to change" kind of attitude about it all. It's frustrating, but when you become a family of sorts, it's hard to be the disciplinarian.

I've been doing what you suggest and I'll continue until I hear complaining.

Almost every choir director faces this dilemma. Two things:
1. have the weakest ones sit and sing next to the strongest readers. Some will rub off.
2. put together a sight reading session for the summer. I did this last summer and the novice and the know everything got something out of it. I can give you a link to all the music if you want. Here is the outline for that event:

I instinctively dislike situations which discourage those who can give more from giving more. As I work with children, it is always the case that there are some who are more capable than others.
I would suggest:
Make the ability to read music a norm for the choir. Offer training at every rehearsal. No compulsion for this - just some quick short lessons for a variety of levels, from total beginners to those who can competently sight sing but all need to practice this skill. Let the ones who are resistant see what can be achieved when you can sight sing.
Have you a place for 'special 'pieces? This could be at a particular point i.e. offertory or for a particular feast ie Christ the king, Christmas etc.
Subdivide the choir into groups who can each have a special piece to learn - from a simple piece for the less competent, to a complex piece for the more able. Your able singers should be able to quickly learn and come alongside for the simpler pieces, as well as having their own more complex pieces to rehearse. That way you can get the more able to do more satisfying and challenging pieces without leaving anyone else out in the cold. As people improve in their singing, they can be given pieces that challenge them more.
you might for example present four or five pieces to the choir at the beginning of the season - specifying the level of competence needed for each piece, ie This is a piece for non readers, it is available to listen to , so you can learn it at home by rote, here is a piece by Palestrina, you can listen to the audio, but you will need to be able to follow the music to know how your voice fits in, etc.
Let people choose for themselves - but be clear about the standard for each piece and don't relax it just because someone likes the piece and feels they could do it.
Give choir members the ability to move on towards more complex pieces as their sight singing improves over time.