Although I’m a firm supporter of free software
Unless I'm misreading this, your argument seems to be that software freedom is irrelevant in the face of technical superiority or popularity. That's exactly the opposite of "firm support" in my view.
I'll offer a counterpoint to the "best tool for the job" thing: before git existed, Linux development relied on a proprietary VCS called Bitkeeper. Licenses for Bitkeeper were "graciously" donated for gratis by the Bitkeeper developer. Andrew Tridgell, who was not party to the Bitkeeper EULA, telneted to a Bitkeeper server and typed "help". The Bitkeeper developer, in retaliation, revoked the Linux developers' gratis license to use the proprietary "best tool for the job." This was what forced Linus to develop git, which became the most widely used VCS in the free software world. (read: Thank You, Larry McVoy by Richard Stallman)
Proprietary tools can seem to be useful in the moment but developing a dependency on them, and encouraging their use, is dangerous. Discord might seem like "the best tool for the job" until it enshittifies, just like its predecessors did, and just like its successors inevitably will. We've seen it happen often enough.
I prefer to use, where possible, the term "software freedom." This keeps the focus on the four freedoms enjoyed by the users.
If I need an adjective, I'll prefer libre, then free. "Libre" has the disadvantage of not being a native English word, but it has relatives such as liberate or liberty, so it's not too much of a stretch. "Free" has the disadvantage of being misconstrued as meaning free-of-cost, but this can be explained away.
"Open source" is problematic for several reasons. Firstly, "open source" is just as easy to misconstrue as "free" is - it can be misconstrued as "you can look at the source code" and many companies and organizations actively take advantage of this misconception. Also, "open source" puts the focus on source code, with the idea that having more developers with access to the code makes the program better technologically. This claim is debatable, but putting the focus on the source code makes it seem like open source only matters to developers and is simply irrelevant to those who don't have the want or ability to use the source code. However, even non-developers can avail themselves of freedoms 0 and 2, and can hire other people or make use of the community to exercise freedoms 1 and 4. Sometimes, open source is misunderstood to refer to the community-based development model that is more properly known as the "bazaar model."
"FOSS" and "FLOSS" are problematic for similar reasons as "free" above: they are very easily misconstrued to mean "free of cost and also open source" (whether open source means actual open source or simply "can view the source code"). It's not uncommon for something to be described as "not OSS but free" for example.
For non-libre software I tend to just say proprietary. I'll sometimes say non-free or non-libre, but non-free and closed source have the same problems as free and open source.