NixOS in it's current form does have it's limitations but it's ever improving. I personally have never had issues doing what I needed to firewall wise, but I've not done much of anything complex. Mostly just opening ports and a little port redirecting.
I've never used stow so I can't speak to it specifically. Home manager is nice for two reasons. If you're already using NixOS you can have one unified config for your whole system. And because Nix is a programming language generation these configs, you may be able to do thing you wouldn't otherwise. It also has some nice defaults that you may not get without.
Because your Nix config also configures your software, not just installs it. Admittedly, with base NixOS that's more true with server software than desktop. But with the addition of home manager you can also configure many desktop apps in your Nix config.
The #nix:nixos.org room on matrix is a great place to ask questions if you get stuck.
They're both good DEs with their pros and cons. I'm glad you found something you're happy with! For me that's Gnome but I've used Plasma 5 quite a bit two and it's a close second for me. I don't think there's much use in bickering over which is "better".
As others have said drive LUKS is primarily meant to protect against someone getting your data if they physically steal your device/drive. This is less of a risk for a home server then say a laptop or phone.
As someone who's bough several albums on bandcamp, I'm pretty sure I've given more money to actual artists than the average spoify user ever has through their subscription.
So making an app when you've never programmed before is a big undertaking, but not impossible. You mentioned termux, so are you looking to make a command line app? That would be the easiest place to start. A simple cli dice roller is a great beginner project.
Beyond that, how do you envision the character sheet part working?
I quite like Rust so I'm biased. But I understand it's not always the best choice.
Actually good and not pain in the ass to set up FOSS STT? 👀
I generally prefer podman to docker at this point for the reasons you stated. However, podman is not 100% compatible with docker, and I have run in to issues with a few tools, that were admittedly poorly written. Mostly around how they deal with file permission when move files in and out of containers.
Wayland if possible because it generally performes better and is actively maintained. Xorg if Wayland doesn't support your system yet.