For FF1 (script-wise) absolutely.
But I would (mostly) compare the other FF NES retranslations to Choppasmith's Mobile-Script Port of DQ1.
The other FF retranslations utilizes names, terminologies, and parts of the script used in later ports. But it leaves out new content of script specifically made for said ports.
Any content from the original game that did not make it to the remakes were retranslated from scratch.
Yep, that's pretty much how it was done. FF2 and FF3 were done by looking at the GBA and DS remake, respectively. The 2016 version of the FF3 translation (the one that was 1mb) had essentially 75% of it as a script port, and the current version is a reworked version of that. It's hard to describe the exact process of how I did them, but basically I coded tools to dump the Famicom scripts, then looked at the respective NPC's line in the remake version and removed any extra stuff that was added. In some cases I even checked the Japanese remake version on YouTube to see if the text changed between the original Famicom and the GBA/DS remake if I wasn’t sure of if a detail was added due to a script revision or just localization smoothness.
Now, FF1 is a bit of a different case because Square(-Enix) made revisions to the Japanese script twice - once in the WSC/PS1 version, and again on the GBA version, and the revisions made to FF1 were more drastic than FF2. The GBA version that was released officially in English is a translation of a heavily revised FF1 script so it's not exactly usable/abridgable to the NES like the others were because the English was written in a way to make the information presented in the Japanese GBA script flow nicely (FF2 was doable because the changes weren't as drastic).
So out of the whole trilogy, FF1 is where I'm doing an actual retranslation. Not sure if I can claim "from scratch" since I'm still
looking at the GBA English script (as well as the original NES localization), but this isn't an edit of the remake script like the FF2 and FF3 ones were.
I completely agree with ChickenKnife and NeonStreetlight. To me, the appeal of this translation is to experience the game closer to its original Japanese developers intended.
Chaos Rush, you were especially concerned about the opinions of those who grew up with FF1. Well, I was 9 years old when it came out, and I had been highly anticipating its release since I was a huge fan of "Dragon Warrior". It's the first game I ever saved up for and purchased with my own money (mostly earned from mowing lawns), as opposed to receiving it as a gift. I played through it and loved every grindy unbalanced minute. So it was a very special game for me during my formative years.
That said, not only would I not be upset at you changing the "Four Fiends" title, but I'd be happy to see it changed to something that's more accurate. Hell, I woudln't even care if you got rid of "I, Garland, will knock you all down!" I've already experienced the official US localized version of the game--many, many times. Your mod, however, offers a chance for me to experience the game in a new more authentic way, and that's what appeals to me about it.
I appreciate you all voicing your opinions on this matter. I may consider making an alternate version where I make my own localization choices (Chaos Four instead of Four Fiends, "Aquair" instead of "Oxyale", no meme-Garland). I still want to make a version that adheres to canon though, because even though this will never happen, I would like to make something that Square Enix would approve of.
Here's a screenshot of my current point in the translation:

I've revised the FF4 font a little to make it easier to read for single-spaced lines (letters like p and g don't touch the next line anymore).
To prove I'm not talking out of my ass about this being a retranslation of the original text (not that anyone doesn't believe me),

Original NES line: "I feel fine now.Thank you."
Original FC line:"私も この通り 元気に なった。ありがとう....クリスタルの戦士たち!" (literal: "[me][in this way][became well].[Thank you]...[Warriors of the Crystals/Crystal Warriors]!)
I'm just using this as an example of how I'm looking at the original text, to the point of incorporating odd places in the original script where they couldn't decide if they were the Light Warriors of the Crystal Warriors. (I'm wondering if I should just straight up write "Crystal Warriors" lol)
And here is my new text editor in its current state:
