Agree totally on the hills comment, that’s where you need some firm regroup rules if the parcour is going to be a struggle for some people. That’s why route choice probably is quite tough sometimes.
If I think of one of the rides my club does, “Fast Friday”, this is a chain gang, through and off ride on which I’d say you’re looking at 3 w/kg to over 5w/kg for some people. It’s what we would call flat but there are some drags which can cause splits, however the biggest cause of splits is people going too hard, this is why the ride protocol / emphasis should be made clear at the start. “Let’s keep it together”.
This will mean that the faster people will often be chatting before their turn on the front while others are working hard continually, but with our group there are enough fast people to make it hard enough, the slower ones can just skip turns but still hang on. Admittedly it doesn’t always go to plan (people can also have off days!) but my point is you can cater for all.
A final point to add, as I mentioned before, the fast people should not treat this as a key training ride when the group is mixed, treat it as helping your club mates out, showing them how to keep it together and encouraging people to step up to improve themselves.