VM are as easy as point and click with GNOME Boxes, also available as standalone.

All those distros are compatible with grub, and come with their own copies of it. You just need to install your distros, and then when you say "I want THIS ONE to manage boot", you follow this tutorial. (It's supposed to help you reinstalling grub after Windows, but it works fine for grub after another Linux instal).

Or, if you want to be lazy - install last the distro that you want to manage boot, then tell it "screw the current boot, reinstall it".

I wouldn't bother with a bootloader partition. The bootloader runs fine from any distro partition, and it's small enough so you don't need to worry about it wasting space.

swap

I've been running my system without swap whatsoever for quite some time, and it runs fine. But if you're planning to use hibernation or similar, reserve the same amount of swap space as you have RAM; for example if you have 8GB RAM then at least 8GB swap.

IMPORTANT: if hibernating a distro, don't boot another distro, otherwise the hibernation data will get wiped.

Grub is compatible with pretty much everything.

What?

1. Install virt-manager qemu qemu-kvm
2. Run virt-manager
3. Install a new distro, choose the .iso that you downloaded, assign 8GB RAM and 60GB storage
4. Leave the rest default
5. Follow the Distros installing process as usual
6. Delete the VM if you are done

Important note: using distrobox or toolbox you can run packages of pretty much any distro on your Laptop. I am currently using Ubuntu PPA VLC 4.0 on Fedora Kinoite.

Fedora's swap on zram shouldn't pose a problem - at most it won't use the disk swap, but other distros still would.

Encryption is important but I wonder if OP would make much use out of it, given that he plans to bulk store his items in the cloud. The storage partition would be mostly for things "at hand". And if necessary, as you said, some elbow grease lets you have encryption and still access it from all distros.

I don't recommend OP to mount that partition directly to /home itself. It's bound to create problems later on due to software in different versions interacting with software that may or may not be present depending on the distro. Mounting it inside some other directory (even inside /home, e.g. /home/username/storage) feels considerably safer.

/home is a directory ("folder"). Inside that directory, there's one subdirectory for each user, where you're expected to keep your personal files, configuration files, user data from software (e.g. save games from your games), stuff like this, personal data.

And Linux allows you to mount any partition in any directory of your choice.

Because of those two things, a lot of people create a partition and mount it as /home. So if something bad happens with the distro, and you need to reinstall it, ~~your princess is in another castle~~ your data is in another partition, safely stored. It's usually a good idea, but in your case it's a bad idea - because your /home/[insert username] directory from one distro will be likely the same as in the other distros, so they'll interfere on each other, and software user data will become a mess.

Instead, what I recommend you is to not create a /home partition. Let each distro have its own /home. However, do create a partition to store your data, and mount it inside your /home/[insert your username] directory. That way you can access all your files from all your distros, but the software user data won't be mixed.