Can you break down New Threat a bit more? Just curious for when I eventually play OG VII. While I'm not quite confident enough to call them straight up superior to vanilla or that there may not be better mods that do the same thing, I always recommend Ragnarok for VIII and Struggle for Freedom for XII whenever those games come up. Really tough difficulty wise while not fluctuating or becoming so bad it encourages grinding. I had solid experiences with these two 'black sheep' games and I am confident the mods played a major role in that and I'm going to do the same with any future FF games I play as well.
LandonRay's mods come up quite often but I've never finished one and they seem to veer more into the frustrating and imbalanced side of difficulty mods. Then again, when numbers tweaking is all you can really do, the means for creating a quality mod are limited.
I've played New Threat a couple of years ago and there have probably been updates since. I'll sumarize its strengths as follows:
1. Overall challenge from tougher opponents with more complex AI routines. This ties into everything else, as now you have to think about what you're doing. It's not hard enough to be considered "hardcore", but it forces you to always have a battle plan ready. No grinding required.
2. True character diversification. Instead of being blank slates with negligible stat variation, now you get a choice in how to develop your characters. You can turn Cloud into a pure melee fighter, balanced or pure mage, for example. You can turn certain characters into glass cannons, tank mages, offensive mages, etc. exactly in the way that is implied by that characters depiction, or subvert their stereotype in interesting ways. Having specialist characters makes all characters useful and you may switch one for another in certain areas or to try out new strategies.
3. Proper itemization. Instead of the usual routine of buying new improved versions of your characters' equipment each you time you get to a new town, New Threat makes every piece of equipment somewhat unique, with its own set of advantages and drawbacks. You'll keep old equipment for a long time in some casess, due to the way it fits your strategy.
4. Recognizing the potential of the Materia system. I'm not sure how much you know about it, but in the OG game what you do is attach 'Materia' (these glowing orbs imbued with magical power) to equipment in order to cast spells and gain attributes. You can also combine them if you have dual slots in some of your equipment. Due to all the other changes, now you are forced to come up with your creative combinations, as you can't just spam Attack and expect to win. The effects and stat changes of the materia are also more significant to allow for more radical character variation.
5. Miscellaneous: QoL changes for cases where you'd have to backtrack long distances; some (very subtle) changes to the script to make the rather convoluted narrative more understandable (I don't usually approve of such things, but here it was done quite well).
There are probably more things that I'm forgetting, but I that's the gist of it.
Regarding LondonRay's romhacks, it's been quite a while since I've played those. There weren't many alternatives back then (I don't remember hearing about Ragnarok, tbh). I wouldn't recommend playing the FFVIII one unless you're a masochist and truly love the game (as I do for several reasons, but mainly out of nostalgia). I imagine it's much harder to balance than the others, given how unbalanced the Limit Breaks are. His FFIX mod I found hard but fair and I'd recommend it over vanilla for non-casual players. I haven't checked for any new mods in the last years, though.