I main ubuntu 19.04 (about to upgrade to 20.04 LTS SoonTM), flipped into Kubuntu Desktop Enviroment (KDE)
I have my install on a 500gb NVMe SSD, and a few Seagate 2TB 7200rpm HDD's for games and bulk storage.
I also have a 200gb HDD which has my windows install on it, and another 2TB HDD for windows to run and play with.
I can Read and Write to my Windows installation and secondary disk formatted in NTFS with no extra steps, Other than clicking the device in Dolphin (Or whatever file browser u prefer), and the system mounts that device for use. I from time to time need to use windows for stuff that unfortunately cannot be handled on Linux yet.
For obvious reasons, I CANNOT read or write to my Linux install (or other linux fs disks) on my windows install. Even if I use WSL. I think more accurately, WINDOWS isn't a big fan of Linux/etc filesystems. Linux in my experience has never given me a problem with any common filesystems I've thrown at it.
I copied my photo/video directly from my win install to my linux install. Things like music I mainly use spotify for, and anything that was local on my drive was moved over without issues. Out of box, linux reads most formats. If there is some special format though, likely there is something you can either apt-get in or build from src.
As simon said, things like steam games DEFINITLY should just be freshly installed.
If you plan on gaming, I would check out Lutris, which can add even more compatiblity than just proton/WINE alone. There are also some discords focused on gaming on linux worth checking out.
You will find it a little tedious sometimes to get games to launch when not native, which sometimes is as simple as editing some settings and going from there; but it can get difficult, luckly there are resources that help.
Another thing to check out is ProtonDB.
Matrix and Simon bring really good points, but I think some advice that is more appropriate would be that Your milage will vary.
As for cloud storage, it is an option, but I would error on the side of caution and make sure you have a "Cold Disk" for critical files locally. If it's work or professional files and apps, I always try to keep BOTH a cloud (offsite) and local copy.
Storage devices are in an interesting meta right now, they are sorta waiting for the next big innovation, as we are creeping VERY close to the point of diminishing returns. Like, we are selling HDDs with like 30 tb of storage, and NVMe SSDs with literally 8 tb of storage.
One final point. MOST popular distros have liveusb installations in with the installer, so you can boot and try out the OS without actually installing the OS. If you are unsure about your choice of distro, this is a GREAT way to check and see how it looks and feels. You can even address some compatibility problems in advance so you know how to do it on the actual install.
Edit; I am using btrfs for my installation disk, and LVM2 for my secondary (it was once my main disk, that's why LVM)
If you have physical drives for a hefty transfer of videos and music, it's the fastest and easiest method really.
