This kind of thing is brilliant, great idea!
Having some variable secretly controlling when you edge, or ruin, or cum or switch from penetration to clit for example, makes everything more exciting.
It can be anything from a dice, to other people’s actions, or random events like what’s on TV or busses passing the window… Anything you can predict a little but not control.
You can even make your own denial wheel with whatever options you want!
As for learning to cum from just fucking, remember, if you’re hitting that g-spot right it often feels like you want to pee – that is how it is SUPPOSED TO FEEL at first. Keep going with it and it’ll feel better and better. Also in learning to cum from there, use your vibe to get you close and then switch just to the dildo to try and finish. The general arousal from your clit will help carry you to the edge in many cases (but not always).
And finally, remember the majority of women do not cum from vaginal stimulation alone (although I think that’s mainly bad technique or lack of experimentation). But definitely for some women it just doesn’t work, so don’t worry about it, just have fun!
Oh and finally, if you plan to start playing with your clit THE MOMENT your friend brings up the chosen topic, you should probably warn him in advance 😉