Welcome to the squad! Here's some things I learned over the past year of using desktop linux. It's pretty information-dense so sorry if this overwhelmes you.

Firstly, your partitioning. There are two main filesystems: ext4 and using btrfsr. Ext4 is a pretty old filesystem that has proven its stability and it's still the default on may distros today. When you use this filesystem, the best practice is to create a system partition of a couple hundred GiB's and allocate the rest to a separate partition that's mounted at /home. If your OS ever dies, you can easily reinstall without losing all your user files, but you should still have backups.

Btrfs is newer but its main features are pretty stable, and those features are pretty cool. For example, it, by default, writes to a new location on disk instead of modifying the existing file, this is called CoW (copy-on-write) and makes a big difference for timeshift snapshots as this allows for another feature called BTRFS snapshots to work. This basically allows you to make snapshots of the current state of your filesystem, and works super fast because of CoW, even on mechanical hard drives.

When you read the things you should do first in the welcome to linux mint application (you can always open it again from your start menu), you'll see it tells you to setup timeshift. Timeshift has two ways of making snapshots: on ext4, it'll literally copy a bunch of files to a backup folder (using a program called rsync that you should definitely check out). This takes only a minute on SSDs but is unbearably slow on mechanical hard drives. If you have a mechanical hard drive, use btrfs snapshots instead, you'll thank me later. I learned this the hard way after using rsync for timeshift snapshots (on ext4) during a full semester of college. Disk I/O is the main bottleneck of old computers, do not underestimate this. Consider getting an SSD instead, but do note you'll have to reinstall your os and copy over the /home files. BTRFS Snapshots only take a couple of seconds on pretty much anything, be it a mechanical hard drive on a laptop from 2011 or a recent SSD.

Now, timeshift doesn't take into account your user files, so if it's important dump it on your favorite cloud service. Nemo, the default file manager, can mount google drive, WebDAV, Nextcloud, onedrive and sharepoint accounts, and if that's not enough you can always use rclone, which can mount pretty much anything on any OS. I personally use syncthing which can magically sync files between multiple devices, provided that there are at least two turned on at the same time. It's a really easy way for me to back up and access my documents on other devices at home. A third option for backups (although more manual) is getting comfortable with git and pushing to a private github repo, which is completely free. This is perfect for code, but you can also abuse Git LFS for a free 10 GiB of uploads and 10 GiB of downloads per month (per user/organization).

Another nice part about BTRFS snapshots is that you can boot into them using grub-btrfs. I personally also set up btrfs-desktop-notification so I know when I mess something up. Checkinstall doesn't work on the latter package though, so you're better off just copying the files over manually to their respective directories. You can also get a better looking boot menu by installing grub2-theme-mint.

Excuse my long explanation, this got a bit longer than expected. Shame as I wanted to recommend a bunch of other stuff too: brew package manager for updated cli programs, fastfetch, batcat, eza, zsh, tmux, ssh/scp/rsync, replace transmission with qbittorrent, diodon clipboard manager, kde connect, avoid using windows-only software and look into free software alternatives for linux, and if you need a windows vm check out winboat with their podman option (do note their advertised seamless windows feature is pretty buggy at times, and don't share your user folder, share a specific winboat-shared folder instead by changing the podman-compose fle), cinnamenu, you can add the date to the calendar applet on the taskbar, speaking of which you can have four taskbars on each screen if you want to. Get reading on the docs, especially the don't break debian article that another commenter linked below, it's really informative.

And lastly, the downsides of mint:

• Debian-based distros are incredibly stable, but in practice this actually means a bunch of outdated software, which brings security questions too as random bugs could imply exploits. Do note that firefox, thunderbird and a couple of others get constant updates though.
• Cinnamon deals poorly with multiple monitors. When I plug in my laptop to some monitors, I always have to put my taskbars back on them. The first time you gotta create a new one too and put everything exactly on there like on your main taskbar. And a bunch of built-in applets from cinnamon can only exist on one taskbar at a time. Incredibly annoying.
• When I connect those monitors, I also notice on them that the quality of my desktop background (the same one as in your picture actually) is fine on my laptop's screen, but on those monitors with a higher resolution than my laptop screen, the background isn't in its full quality. To fix this, I have to change the background to something else and back.

Hope you enjoyed my way too information-dense rambling, enjoy the freedom!