Linux

First thing I'd do is ditch the GUI file manager: get comfortable with cd, ls, mv, rm, etc.

After that, maybe start with basic text manipulation, like grep, awk, sort, uniq, etc. This ties in nicely with IO redirection, which is essential for a "CLI based workflow." Get comfortable with pipes and file redirection, it's extremely powerful!

Writing shell scripts is another super useful exercise: any time you find yourself running the same set of commands multiple times, think about making it a shell script. You may end up with some really useful little custom tools that way.

KDE is your friend in the trenches, Kubuntu > Ubuntu 100%

Regardless of whatever distro, definitely keep all your stuff backed up on an external drive. I had to "restart" my install a few times to get the hang of things. Yet I still keep all my info on an Vegacrypt encrypted external just in case to this day.

Welcome fellow Linux beginner.

What are some really important basic commands I can use to start branching out into Terminal command structures

Since I refuse to touch the Terminal like my life depends on it, I can't help you with this 😅

And if Cinnamon has shortfalls or weaknesses that I may run into eventually, what are some good alternative distros that I could leapfrog to eventually?

I'm a little biased, so take it with a grain of salt: Cinnamon's biggest flaw is the lack of support for the Wayland protocol. It's still running with x11. As far as I have read about it, X11 has several big security flaws, and it slowly dies a long, quiet death. As far as I know, the Linux Mint devs (makers of Cinnamon) haven't even started on Wayland support. A good alternative would be the KDE Plasma Desktop. It's insanely customizable, and the default look comes very close to classic Windows. As a KDE Distro, I would recommend KDE Neon since it's based on Ubuntu too.

Here's a good reference: https://linuxjourney.com/

Dotfiles. They let you customize your command line experience. There are loads of examples on GitHub. Reading through the comments on dotfiles that other people share help you learn about all the things that can be done. Customize your shell with aliases, functions, history behavior, syntax highlighting, etc.

I'm a Mac user who tinkers with Linux off and on, but I absolutely love that you're learning about the world of Linux. Welcome to the party!

https://github.com/topics/dotfiles

Shameless plug of some of my own custom tooling: https://github.com/fantismo/dotfiles

Remember to play. There are multiple terminal emulators, window managers, shells, all the things! Try 'em out. When installing stuff to test via distro package, keep note of which dependencies were dragged in so you can get rid if all of them, not just the thing you wanted to test. Also keep a list of what you tested and later got rid of (and why) so you avoid testing it again.

If you ever ssh into other machines to do stuff, learn the minimum of vi (classic w/no arrow keys, not vim) and bash. Servers "always" have those (though FreeBSD also has tcsh).

I do not know what sort of power management software exists by default on Ubuntu, but for laptop use I would strongly recommend getting a power management package like TLP to configure power profile settings for your laptop when on battery and on charge. It can greatly improve battery performance. Some alternatives like auto-cpufreq and powertop exist, but I have tried all 3 and found that TLP worked the best for me.

Probably already mentioned in other comments, buy it can't be stressed enough: Backups !

Part of learning to use Linux is breaking it. You will make mistakes that will bork your computer. You can either spend hours to try to fix it, or you can wipe everything clean, reinstall, restore from your latest backup, and be done like nothing happend in no time. (Maybe you want to go the hard way for learning, but it's always more relaxing to know you also have the easy option at your disposal)

Ubuntu comes with Timeshift, make use of it. Also plan to make backups to external storage or NAS, in case things go real bad.

I think the two "major tips" that I can give you are simply

1- Package manager is your best friend.

2- Figure out the "know-hows" of Linux (i.e who "is responsible" for the video card, who deals with the cpu, how do i configure my sound card, how do i configure my video card, etc.).

Master those two tips and you can call yourself an average linux user.

Set up btrfs and timeshift. You never know when you need that big undo button.

Most distros are good enough without much tweaking. in terms of commands...some more advanced stuff...

start poking around proc to see what linux gives you, without needing a program to get it!

Honestly, youtube "basic bash tutorial" and watch/follow along with a video or two. Helped me a bunch.

Books, books, books! O'Reilly publishing is your best friend. Search engines are next. And finally, Youtube.

Unless you are looking to work on shared systems/servers as sysadmin or other jobs, explore shells like zsh or fish and customise them rather than stick to bash. A lot more user friendly and accessible advanced features. Helps with learning a lot. Zsh is compatible with bash but fish isn't. So choose based on what your goal with learning shell is.

If you are sticking with debian based distros, try apt and synaptic(GUI) to install your software. At some point you might also need to install tar archives. Don't get worried as most guides should be easy to follow.