I think the RSPCA and AWL type big shelters are frustrating for those of us who want to help the animals directly.
It would be so frustrating to sign up for dog walker/trainer and then find that pooch you put all the work into - gets PTS for one reason or another (eg being rehomed to a do not care family then returned ruined)
I think that's the hardest bit about being with the big shelters, they have to put a lot of dogs (and even more cats) to sleep.
There's a few smaller rescue / fosterers in my area that I think I'd have an easier time working with - one dog at a time.
But not until I can reliably train my own dog.