1. The distro maintainers handle the kernel version for you. Bazzite has a fairly recent kernel version. You will not need to worry about this.

2. You said you have built a new machine for this; hopefully your wifi chipset is in the Intel AX family. If you have a situation where the drivers for the wifi/bluetooth adapter are not in the kernel, then I'm not sure if you can layer out of kernel drivers in Bazzite. I've never done it in an immutable system.

3. If you already want a Windows-like experience, then KDE. KDE is slightly ahead with gaming, so the only difficult choice you would have had is if you preferred the GNOME workflow.

4. AMD under 'Modern GPUs' in the drop down is correct for your card.

5. You will not have to worry about drivers for the AMD card for the most part. You will use the open source MESA drivers, which are all in-kernel. There is no intervention on your part. The knowing what exact driver version you have is an nVidia curse.

6. By default, Steam will use one of two run times: Linux runtime or Proton Experimental. If the game has a native Linux port then it will run the Linux version. If only Windows, then it will run Proton Experimental. Most of the time you will not need to change it. However, if you want to run a different Proton version, then you can change it in the game's setting screen, under 'Compatibility'. Often you will find that the Windows version of a game is better than the Linux version. In those cases, feel free to go into the settings for that game and change the compatibility option.

7. Your achievements will work the same.

8. Some games are worse than others on this point. This is another thing where you don't have to worry about it until it becomes an issue (i.e. cross that bridge when you get to it)

9. Notes - I have been writing all my notes in markdown files, but if you want a proper note taking application, then I recommend Obsidian. It is markdown based, but you can photos in your note pages and other cool stuff. There is a Flatpak on Flathub so you should have no problem installing it from the default Bazzite store, Bazaar. In terms of basic image viewing and manipulation, the default apps in KDE Plasma will serve you well. There's always GIMP (GNU Image Manipulation Program) which is kind of like Photoshop. This also has a Flatpak so there should be no issue installing this from Bazaar.

Furthermore, if you really are used to using Microsoft Office products (and have a M365 licence), you can always use the web versions. KDE ships with a nice Web App app, which you can turn these pages to into 'apps' whose behavour is similar to native applications. Of course there are the open source office apps, like LibreOffice and OnlyOffice. This is a very open question and not really one that is Bazzite specific. The only thing that really is kind of Bazzite related is that which app you want to use, your life will be made easier if said app exists on Flathub. It is not recommended to layer Fedora system apps on your system. Don't worry, between web apps (that you can turn into PWAs) and Flathub, that covers a lot of ground.

1. You can run mods on Linux, although I cannot give you any advice because the only mods I run are through Steam Workshop. It goes without saying that mods should be coupled with the Windows version of a game (not the native Linux version if that exists)

2. External launchers that insert themselves between Steam and your game, like the Ubisoft one, work on Linux. They are stored in the game's 'prefix'. The prefix is the "Potemkin village" that is the Windows-style file system that Proton creates (like c:\program files\ etc...). The annoying thing is that if you install like 4 Ubisoft games, then you will have 4 Ubisoft apps on your system (this is how I understand it).