but more so on level of directness we would prefer
If you ask me, I believe most yuri fans and very likely most Nanoha fans would prefer a series about NanoFate cuddling all day than whatever we're getting in the new Vivid Strike anime, but we know that is not the niche of seinen, a Sakura Trick 2.0 would be hard to sell too.
You are probably right. I would simply prefer something good, but it does not look like we are getting that with Vivid Strike. For the record I do not think Vivid is that great either.
they could just as easily be described as "really good friends" and it would not change much
For that I believe you will have to at least change their permanent living arrangements and dedicate a few pages to them getting a divorce and negotiating the custody of their daughter.
Fate seems to mostly be away for work (and friends can live together), they did not dedicate any pages to them getting married either, and they were both a mama to Vivio in Strikers regardless of any official custody status. Look, I am just playing devil's advocate here and looking at this from a perspective of someone who has always looked at Nanoha and Fate as friends, and continues to do so. I think it is a plausible scenario.
I continue to believe that a certain level of vagueness on the topic is very much intended
That has been the case since the first anime, Nanoha has never shown explicit romance on screen for any of the canon couples, even the straight ones, and this is very common for most anime outside of the romance genre when you think about it. Nanoha and Fate are by far the couple with the most on-screen romance in the franchise even if it's still not overly explicit, and this is all in a big part only because they're the couple with the most combined screen time through all series, not because Masaki Tsuzuki wants to move to the romance genre.
Okay we are going in circles with this, but I am not talking vagueness as imposed by genre. Yes you are absolutely right, Nanoha is not a romance, and as you say the relationship will most likely never be addressed directly. But do you really, honestly believe that is entirely because of the genre, and not because the creators do not want to commit to it 100% (again, for various possible reasons)?
It is not like I am expecting sloppy makeouts in every scene. Here's a thought, why not make Nanoha and Fate wear rings? It would be subtle yet effective, not putting romance on the forefront yet still making things crystal clear. It would not change characters dynamics, nor would it distract from things the story actually tries to focus on. But they will not, and it is not because Nanoha is not a romance story.
For example, I could see Love Live fans use that for Maki and Nico (just to pick an obvious example)
I'm not so familiar with Love Live, but I guess the difference is that Maki and Nico are not a canon couple, there are feelings hinted and at most you can say feelings of romantic nature from one or the other are written as subtext (though I probably wouldn't agree with this last statement, but that's just my opinion), but there isn't a relationship written as subtext or text.
Yes that about sums it up. My point was only that people use the word subtext somewhat haphazardly.