[...] ask for the opt-in / opt-out once in the main menu for the people who haven't made a choice yet. Show it in a popup over the main menu [...]
This is what I intended to suggest. At the very least, pull the option out of the settings menu and put it in a check-mark on the main screen. It feels ...

I've uploaded my modpack to dropbox here. I'm currently in a discord room, and my Steam handle is Tyshear. Get in touch!

I'd like to start a new multiplayer game with mods. Below is a list of my mods, but any below the first three are negotiable. Ideally we'd get at least three people in the game, but just us two is cool, too. If you want to get in touch, join the discord channel at https://discord.gg/xgqAwkV.

Mods I ...

I don't mean to belittle your experience/request, only to offer a suggestion in the hope that it might be an adequate interim solution.
There's a mod (https://forums.factorio.com/forum/viewtopic.php?f=91&t=12447) which adds artillery emplacements, the largest of which has an explosion radius of 30 ...