Well, thing is, it's not as much subtext as it is Nanoha and Fate doing a sucky job at hiding their relationship. There's no(t much) blushing or awkward situations because of lingering feelings or subtle romance as in most subtext shows, Nano and Fate are already like 3 steps further.
I heard the two movies really push the subtext so watch them if you're gonna do it for NanoFate.
There's still a lot of subtext in all of the Nanoha adaptions. For ecample in the A's manga it said
We were definitely attracted to each other to become "friends"
Like, I didn't even add the quotation marks. That's pretty much the manga saying "Friends. Yeah right."