EDIT: Last paragraph seems to be specifically what the OP is looking for.
I keep important files and projects backed up with Windows' built in File History. As soon as the external drive is plugged in and detected, it copies the directories from the system that you want backed up automatically. Bonus points if you use whole drive encryption on this so it's password protected if you lose it or someone comes in and grabs it.
Keep a list in a text file (on that drive or on internet storage mentioned below) of what programs you use and their license keys/account information that you need to keep if you were transferring to a new system. Bonus points if this file is password protected as well for obvious reasons.
A more efficient method is if you have a majority of things backed up in the cloud.
Google Photos, a password manager, a Dropbox, and a Google Drive account that you know how to access can keep things in a safe place on someone else's storage in case you have a home fire, flood, or other disaster. This is good for the kind of stuff that you may need to be able to view on more than one system or don't want to lose. This will also make upgrading to a new system a lot less of a hassle, as you don't have to transfer all of this stuff over or worry about where it goes or how many places it's in. Granted, your data is in another companies hands but that's the whole point. It's out of your hands, so you don't have to be responsible for it.
I've used Easeus Todo in the past for changing out system drives in the same hardware, and even the free version works great, but I don't recommend this approach if you're changing systems, as different hardware needs different drivers loaded and different software installed, and if you're going to be using the new system differently you won't need some of the programs that you had before anyways. It's safer and easier to reinstall Windows and your hardware drivers fresh if anything's changed as there won't be any conflicts or extra instability or additional things running in the background.
I also keep a backup of my system drive that was saved with the built in Create a System Image tool as my "just in case". I'd use it like a restore point for all my programs and settings if something bad happened or there was an update that broke things, though I haven't needed it yet.
https://support.microsoft.com/en-us/help/17127/windows-back-up-restore
On Windows 10 it's in the left side of the Backup and Restore (Windows 7) menu in control panel.