Welcome to the Linux world! You made the best choice with Debian (IMO).
First off, as Buffalo said, you're definitely going to want to be on Trixie/Testing. Debian stable is great for an old laptop or a server but for something you're going to use every day you need to be current. I would always suggest using the Weekly Testing Build for any new installs which can be found here:
https://cdimage.debian.org/cdimage/weekly-builds/amd64/iso-cd/ Then choose advanced options --> expert install (don't worry, you don't actually have to be an expert).
Your system will for sure not be as durable as Windows. Windows can be near impossible to kill at times which is a blessing and a curse depending on how you look at it I suppose! So Timeshift is a good shout if you want that sort of thing but in my experience Timeshift can be hit or miss so I'd also agree with the above about just backing up all your stuff and keeping a list of favourite programs and configs. Linux will almost always be faster to reinstall than deeply troubleshoot if you need to get it going again in a hurry. I would never advise moving a drive to different hardware. On Windows or Linux. Windows will at some level try to soldier on through but you would only be inviting disaster doing the same on Linux. OS's are compiled and installed for the hardware they find themselves on. Never move them.
Always get your stuff from the official repos when you can but of course there will be times when a .deb package is all you have. Just try to be as aware as possible what these packages are doing and try not to worry too much. I would say install Flatpak as well (easy instructions on Flathub website). You will often find things that are mainly flatpaks these days. And appimage is a great new choice for beginners since these are little standalone versions of whatever app you're trying out. You just download them, make them executable and off you go. But they will never update obviously as they are fully contained.
Seeing as you are on an Nvidia card (a fellow 4090 enjoyer
) I would offer more advice. Don't use nvidia-driver from the repo or any other location people tell you to try (you can hook up to the nvidia developers repo and such like). You should grab the .run installer from Nvidia's website just like you used to on Windows. This may trigger some people but I don't care. Unlike Windows, Nvidia offer several different versions of their Linux driver. The one in the repos ,535 as of writing this, is old AF and won't offer you things like DLSS etc. So you'll want to pick a newer one. I'm currently on the latest (565) with no issues. Just do a quick Google before choosing to see if there's any trouble with the most recent driver. Before installing you'll need to have all your ducks in a row. Things you'll need:
apt install build-essential (probably should always have this anyway)
apt install linux-headers-$(uname -r) (always good after a linux kernel update)
apt install gcc make acpid dkms libglvnd-core-dev libglvnd0 libglvnd-dev libc-dev vulkan-tools (the above might have already brought in some of these. dkms super important)
Don't worry about blacklisting Nouveau, the Nvidia installer does that for you these days.
Get ready for the 32bit Nvidia libraries. Make a folder anywhere you like to house them and append "--compat32-libdir=*" to end of the ./ execution command on the .run Nvidia installer. Where * is the place you chose for the 32bit libs. If you forget don't worry, the installer will mention this.
Once you trigger the installer you choose Nvidia Proprietary and then pretty much say yes to all the options it will give. Once done just reboot and you should be golden. Updating the drivers in the future should be as easy as just running a newer installer, it's pretty clever these days. This is probably the best write up I've seen on the subject:
https://www.if-not-true-then-false.com/2021/debian-ubuntu-linux-mint-nvidia-guide/ It's a bit long winded and you don't need to follow it every step but it's a good read to understand what's going on.
For games, make sure the drive you install them on is formatted Linux friendly like ext4. Steam and proton can't read anything else. That's all I can think of for now. You will certainly break everything a few times but don't give up hope.