Not sure if it fits what you’re looking for but I usually use YUMI for multi boot situations, can’t recall it giving me any issues over the years. But I don’t do anything overly complex either.
Never had the need to use Ventoy myself so can’t really give a good comparison but maybe others have used both & can give a better review.
PS - For what it’s worth my basic toolkit is YUMI with https://www.system-rescue.org and https://www.memtest.org, that alone covers the vast majority of my diagnostics/rescue situations. But I’ve also added Windows 10 ISO onto the multi boot on occasion which could be useful for getting to a Windows prompt with Windows tools when needed - though I have a habit of keeping Windows on its own USB via https://rufus.ie
Yup there’s at least a few projects that can do in browser file transfer using WebRTC e.g.
https://sendfiles.dev/
https://sharedrop.io/
https://github.com/kern/filepizza
And of course https://github.com/magic-wormhole/magic-wormhole (they used to have a website too but maybe that went away?)