Rsync (or better alternative) to copy and sync select folders on various computers across multiple platforms.

I'm absolutely tired of trying to keep certain folders on my computers in sync. I've used just about every file sharing/syncing service out there (Dropbox, Google Drive, OneDrive, etc) and they always end up with a syncing error after enough time passes. Plus they all require that the files you want synced to be in their special folder like 'DropBox' or 'OneDrive' etc. I don't want to have extra folders anymore. I want my normal Documents and Downloads folders where they are supposed to be. Ok so this is what I want to do. I run a bunch of operating systems (Win 10 Pro, OSX, and many Linux distros). All of them contain the folders I want to sync in their home directory by default. Those directories would be Documents, Downloads, Music, Pictures, Videos, and of course Desktop. Every single OS I use has those folders there by default. And that's all I want to sync. And honestly, it isn't even a crazy amount of files either. We are talking about 1-3GB's of data in all those folders combined. nothing crazy. Ok, so on my end, I have several OS and servers that I can use to be a 'master' for this process. I can make it Windows or Linux. And I can make sure that it will always be online. Whatever you think is best is fine with me. But know that I expect this to 100% work. Not 95% work. I plan on throwing a bunch of files and doing edits at the same time on different computers and OS's to see if it can overcome that. Please, please, please DO NOT BID on this project if you are not 100% sure you can make a full proof solution that will work over many OS's at various locations. Please don't waste your time or mine. I won't accept a solution that doesn't work 100%. I already have a bunch of those right now. I want a final solution that I never have to worry about again. If you can't do that, please don't say you can. I will make sure you do not receive a penny until you provide a perfectly working solution.