Hi C.! Wow - I'm sure you've been through it all with 7 kids so I'm not sure how much help I can be, but here's just a thought -
My niece is almost 4 1/2, is also speech delayed and is also a reluctant potty trainer. My sister-in-law tried all the tricks you are doing with few consistent results. She just learned that kids with speech delays are often late potty trainers as well.
It might comfort you to know that it might be a developmental thing rather than any kind of trick or method that you need to be doing. I encourage you to keep on with all the great things you are doing with the understanding that your 4 1/2 year old son just may not be developmentally ready yet.
Your speech therapist at the pre-school or an occupational therapist may be of help in this area.
best of all things to you and your family!