What's your planning process for big group trips?

My university friends and I (and everyone's significant others, so about ~15 people) are planning a big catch-up trip, which will also be the first time to the country where I live for all of them. Planning has been a little higher friction than I expected, because we want to coordinate travel times to specific cities ("let's spend 4 days in City A, then all go to City B" etc), but people also have specific activities only they want to do (scuba, theme parks, etc) within each city. However, there's way too much to choose from and there's no way we'll do everything that everyone wants to do.

So right now to gauge interest in specific cities and attractions within them, we are just using a private Facebook group where people just make an idea as a post and people vote by liking it, and people can discuss the place in the comments. Things that we have fully decided are just placed on a spreadsheet. This process works but it doesn't feel great.

I've also explored Wanderlog, and I really like the fact that you can easily search for stuff and then place it on a map, but unfortunately its UI doesn't really support "branching" trips where some people will do different things on a given day.

What tools/processes have you had success with when trying to coordinate a big group trip?