What do you recommend I do about disk partitions?

I usually put the OS on a separate partition from /home/user, so if I want to reinstall I can do it without losing my home stuff. Once you've settled into a distro you may want to keep /var separate too, in case it ever gets filled up it won't affect your root.

A desktop distro will take up more disk space than a server one, where you can typically fit a server into 20 GB you might want to set aside at least 50-60 GB for a desktop. And that's just software and package caches, not counting games and such. If you split root and /var then 30/30GB would do.

Yes Windows can occasionally mess with your bootloaders if it's installed on the same drive.

Linux distros will typically recognize each other and add each other to the grub boot menu. Also typically you get a choice of whether you want them to do this during install so you should be able to refuse this from secondary distros and re-generate the menu on your main distro to pick up the others.

Is cloud storage sync straightforward?

I think Dropbox is the only one that has an actively maintained desktop client. But rclone will deal with almost anything else.

Should I just use apt to install software?

No, but be careful what you use, and about apt too. The dependency tree can develop issues if you add 3rd party .deb repos that overwrite native packages. Some repos are curteous (Docker is one of them) and publish packages under distinct names, most are not.

Debian native packages can grow long in the tooth because Debian only releases stuff once every two years (next one in 2025). This is where something like Flatpak comes in; the Flathub offer is tiny (under 3k packages) but it's chock-full of useful desktop apps, in case you need a more recent version of anything. Steam or Firefox will be a strong candidates for Flatpak installation, for example.

I use the normal Firefox btw. Never saw a compelling enough argument for the clones and all kinds of downsides.

If you want a package manager learn to use aptitude on the CLI. It has a menu and everything, just takes a bit getting used to. It's the best there is.

Any other pearls of wisdom?

If you want to keep things tidy I would strongly recommend sticking to native packages for all important stuff like system things, desktop environments, drivers. Avoid 3rd party repos if not well-behaved. For anything you're missing or not new enough try Flatpak first. For CLI, programming languages, servers etc. I would strongly recommend installing Docker from their official repo (it's well-behaved) and installing stuff in Docker containers.

You need a swap but it doesn't have to be a partition. You can make it a file on /. You can also use zram and make it a compressed, dynamically sized piece of RAM and then forget about it. Check out the package systemd-zram-generator.