My experience (which is admittedly many years out-of-date) is that WINE isn't very good at anything except games, because games are what the people who use it and work on it are most interested in. When other software works, it tends to be as much by coincidence as anything.
nyan
Switch to a distro lineage whose package manager builds in the necessary facilities? Someone's already mentioned Nix, and Gentoo has the --fetchonly
switch for Portage which will download (but not install) everything required for a specified package including dependencies, so you can copy all of the files to an external drive at once.
You just said "large". I would consider any project with 10MB or more of source "large". Firefox is certainly large by that standard, but so is openssl. If your standard for "large" is "has at least as much code as Firefox", then according to you, the Linux kernel is a small project.
openssl is a vital part of the web, but it is a small tool
You consider 61.7MB of source code "small"? (That's for openssl 3.3.2, and may not include some rust code that isn't in the gzipped main code package.) I think maybe you need to recalibrate a bit.
A whole bunch of non-user-facing projects providing vital libraries that are largely ignored until something blows up in people's faces, as happened with openssl some years ago. Some of them contain quite a bit of code (for example, ffmpeg, which underpins a lot of open-source media playback software). Among browsers specifically, Pale Moon has been around for years, is maintaining a lot of code no longer carried by Firefox along with a fair amount of original code, and has no cash source beyond user donations, which might stretch to paying for the servers in a good month.
The projects with corporate sponsorship, or even a steady flow of large donations, are in the minority. There's a reason the xkcd about the "project some random person in Nebraska has been thanklessly maintaining since 2003" exists.
Money isn't important. Some complex software is, in fact, maintained by unpaid volunteers who feel strongly about the project. That doesn't mean it's easy (in fact it's quite difficult to keep the lights on and the code up-to-date), but it is A Thing That Happens despite being difficult.
What is important is the size of the codebase (in the case of a fork, that's the code either written for the fork or code that the fork preserves and maintains that isn't in the original anymore), the length of time it's been actively worked on, and the bus factor. Some would-be browser forks are indeed trivial and ephemeral one-man shows. Others have years of active commit history, carry tens or even hundreds of thousands of lines of novel or preserved code, and have many people working on them.
It can share TDE's popcorn.
If you're interested in doing the tech equivalent of a party trick (except that it's less interesting to watch), go ahead and try. You'll probably just end up reinstalling almost every package on the system that differs between the base distro and the offshoot. Harmless, but also pointless, since you could just have installed Debian from the get-go and saved yourself a lot of trouble.
There are a whole bunch of Very Silly Things you can do in the Linux world that aren't worth the effort unless your income relies on the creation of niche Youtube vids. For instance, it should theoretically be possible to convert a system from Debian to Gentoo without wiping and reinstalling. I'm not going to try it.
My primary icon theme and widget style are 20+ years old and not flat in the least. You can still have that look and feel on a real computer if you want it (but you may have to compromise elsewhere or do some extra work). On phones, all bets are off.
Dunno what your issue with that icon pack is, but I'd bet there's a good chance it can be solved with a few file renames or symlinks if you care enough to bother.
Most people are not obsessed monomaniacs with room for only one interest in their lives. That means that most Linux users are interested in things other than free software, so many will choose to "dress up" their computers in ways that showcase their other interests, and may use material that is not under copyleft licenses to do so. If this causes you anxiety or confusion, you may want to speak to a mental health professional. Seriously.
I ended up setting up custom themes for multiple different widget sets to get a true black background. It was easy for most QT variants, not too bad for GTK2, really awful for GTK3 because it doesn't have proper documentation for manual theme creation, and I haven't tried to tackle GTK4 yet.
Because they all need different configs (and the window manager title bar etc. may need yet another one), it's difficult to give suggestions unless you tell us which terminal and window manager software you're trying to theme—the requirements for a Gnome session are different from those for something like fluxbox. Some terminal software even has its own built-in theming support.
On the one hand, diversity is usually a good thing for its own sake, because it reduces the number of single points of failure in the system.
On the gripping hand, none of Ubuntu's many projects has ever become a long-term, distro-agnostic alternative to whatever it was supposed to replace, suggesting either low quality or insufficient effort.
I'm . . . kind of torn. Not that I'm ever likely to switch from Gentoo to Ubuntu, so I guess it's a moot point.