I would just keep the fantasy alive with chat and talk. Use some role play. Keep using the sleeve for a while. When the time is right, suggest to introduce another guy in the picture.
Your quote, "I never pushed her about this, and respected her feelings. Pushing your girlfriend into cuckolding is for me the opposite of what I want from the cuckolding. I want her to be the boss of me sexually, not the other way around." Take your time. If it happens it happens. If not, just enjoy what you have.
You are further along then me, and I have been married for 30+ years.
dave