Any software that is in the Xubuntu repositories will also be available in other Ubuntu derivatives, and most likely Debian and all its derivatives as well. Only the official spins are likely to advertise Ubuntu Pro.
Mint XFCE is a good replacement, but I'm also partial to KDE Neon, which keeps preinstalled software to a minimum and is by far the most performant KDE distro I have tried. I myself use regular Debian, with KDE, though you can choose XFCE during the install.
VLC's file format support is amazing for a project that rolls its own codecs, etc, but it's missing some important features for me on the music front, primarily gapless playback and library management. I generally prefer to use software tailored to my DE. I've yet to find a better video player anywhere though; GNOME Videos and Kaffeine come closest and are a little easier to use, but are still far away from VLC's capabilities.
While this new display certainly seems better (in terms of being able to use 2x scaling instead of my current 1.25x), I'd honestly prefer to have a cheaper option that's just 1920x1280, so I don't have to use scaling at all. I don't care that much how "crisp" text looks.
Not sure why, but a lot of other distros did something just like this in the past (see the comments about WUBI) and no longer do. Q4OS still has a .exe installer though.
Right now I'd say the best open-source DAW for Linux is LMMS if you want to do everything just on your laptop, or Ardour if you want to use external instruments.
LMMS has some shockingly versatile built in synths, including a port of ZynAddSubFX, supports LADSPA/LV2 plugins, and supports using Wine to run 32-bit Windows VSTs. I'm unsure of Ardour's VST support, but it at least supports LV2 plugins. Either of those, if you install them through your distro, will likely include Calf Studio Gear, an extensive collection of LV2 effects and a couple synths. As for ones that run natively on Linux, there's synthv1, samplv1, drumkv1, and padthv1, though I've had trouble getting them working myself.
I've found some good stuff on the Linux Audio Wiki but IDK how up to date most of it is.
Almost all distros can dual boot, so there's little reason to highlight one as being the best for it. Dual booting is a bit messy and risky no matter what you do, so back up your data first and, if you're new to Linux, look up instructions for dual booting Windows 11 and your chosen distro.
Only one I can think of that does anything special with dual boot, off the top of my head, is Q4OS, which offers a way to install it from within Windows using an app. (It also happens to be tailored towards people familiar with Windows XP or 7, so it should be a somewhat smooth transition for a first time Linux user.) I haven't tried this myself, but I imagine it just expedites the usual steps of dual booting: shrink the Windows partition, then install into the resulting free space (or to an unused disk).
If that doesn't sound appealing, just try any distro recommended as "beginner friendly", like Linux Mint. Tutorials should be easy to find.
Choose an open-source license and add it to the repository as a LICENSE file. If you want to require any projects that build upon yours to be open-source too, the GNU GPL is a good choice. If you want to allow proprietary programs to include your library without releasing any source code other than that which is directly based on yours, the GNU LGPL is good for that. If you want to allow people to do whatever they want, even use all your code as the basis of a proprietary program without credit, the Unlicense is a good choice. There are a lot of licenses with different degrees of "copyleft" and attribution requirements in between. Technically publishing with a license file is all you need to do, but there are more things you should do.
Create a README text file describing what your program does, and instructing users on how to compile and run it. Consider including more detailed documentation on how to use it, as well.
Clean up your code and file layout so that it's as easy as is feasible for other programmers to understand.
Promote your project to whoever you think might find it useful!
I loved the default theme, the splash screen, all of the customization options, and how lightweight it was, but it's missing some of the conveniences and polish of GNOME, KDE, or even LXQt and Xfce. Using an independent toolkit meant that none of my apps looked consistent, even after trying my best to find a theme that supported everything, and if I explored the settings beyond a surface level things started looking ancient and clunky.
Definitely underrated, and really impressive for how much they could pack into a desktop targeted at older PCs, but still missing quite a bit.
I have to borrow a school laptop just to do proctored exams, because their "lockdown browser" doesn't support Linux, and even if it did, it seems to do some things in kernel mode, so I don't want it on my system.
Surprisingly, most classes at my university are entirely FOSS based, aside from that one piece of software, an obscure scientific program that only one assignment used, and MATLAB (which is easily replaced by GNU Octave.)
No distro I'm aware of still provides official box sets and CDs. Debian still provides materials for third parties to make them, though. Most of the vendors of pre-burned Linux media have also shut down, but one that seems to still exist (and offers Debian box sets) is https://www.shoplinuxonline.com/ .
Debian Stable, in my experience, can stay online for months, even over a year, with very little attention, and still work as well as you left it. You can also install RHEL or a rebuild, like AlmaLinux, RockyLinux, or Oracle Linux, as a workstation distro.
As for the device, my use case is fairly different so I'm not sure what to suggest. Maybe an Intel NUC, or a Framework laptop.
I've honestly never wrestled with Secure Boot in this way; I usually disable it if it won't let me boot my preferred kernel. From my brief online searches, enrolling your own keys is possible, but that depends on the kernel modules being signed in the first place, and carries risk of bricking devices if not done correctly. So you might just want to disable Secure Boot, or otherwise stick to kernels provided by your distribution.
You're looking at bootloaders, not kernels; you need to enroll the kernel with one of those bootloaders. Usually running sudo update-grub while in the OS will automatically detect and add any available kernels to the default version of GRUB.
Any software that is in the Xubuntu repositories will also be available in other Ubuntu derivatives, and most likely Debian and all its derivatives as well. Only the official spins are likely to advertise Ubuntu Pro.
Mint XFCE is a good replacement, but I'm also partial to KDE Neon, which keeps preinstalled software to a minimum and is by far the most performant KDE distro I have tried. I myself use regular Debian, with KDE, though you can choose XFCE during the install.