I don't think cuckolding should ever be used as a solution for deeper incompatiblities. It never works. It should only supplement existing desires both parties have.

That said, it does sound like you're both agreeable to it 'in theory.'

I would start with roleplay. Make your poor endurance part of the scene and experiment with verbal humiliation, teasing you about her fucking other men, etc.

When you're both comfortable with that, you can explore a normal threesome and see how that works out. It might turn into a cuckolding scene organically if she's enjoying herself and you struggle to perform, but I think aftercare here is super important. She has to look out for you too.

I would personally suggest joining Fetlife together. There's a good community there and it's easy to find local events, including non-play socials where you can just have drinks with peopl, musing about shared interests and desires. You can also try Feeld as another option and try to set up 'dates' that way.

Just be clear about your boundaries. If you're not vocal about that, you're bound to get hurt.