Honestly I think this is a great idea given your stated goal here - there are not many springs that behave like the Halo True’s - though it’s not completely alone.
From my understanding, the HT spring is actually what we’d call a “fast curve” - while I don’t think it’s technically progressive (its rate of increasing resistance is constant), it does have a very different feeling at the top, middle, and bottom. The opposite of a slow-curve spring. Starts light, actuates normal, bottoms-out really heavy.
For clarity, you’re probably more likely to find a fast-curve spring that is also progressive than a linear fast-curve spring, at least when it comes to aftermarket stuff. In fact, most springs with fast curves available right now are also progressive, and primarily sold as “progressive” in the name, with mentions of the fast-curve relegated to the description. Progressive fast-curve springs are going to be pretty darn similar to linear ones, because that big difference between the top and bottom will be about the same if not more; just the increasing weight will pile-up later in the travel rather than steadily throughout.
Because this spring type is so vanishingly rare compared to the rest, I actually keep a whole bunch of Halo True springs around for when I want that kind of feel. During the Holy Panda craze (before you could just buy them), people were discarding leftover Halo parts left and right, and I happily soaked-up any available extras.
As for stock switches, the closest thing I can think of to a Halo True is a Hako True (same designer, long story), and they’re only similar in-theory. The same things were kept in-mind while designing them, but the end product is pretty different - and being a BOX design there’s a bit less franken-potential.
I do think frankenswitches are going to be the most fruitful route here in terms of ticking all those boxes, and there’s already plenty of good suggestions here.
I’ve got the Akko CS Ocean Blues as well as the Lavender Purples; I’d say the lavs have a bigger and more prominent bump and less refinement in the sound (needs more effort to tune for clean sound). Totally ignoring sound, though, I might prefer the purples for feel. I’m with @HungerMechanic that they’re at least worth considering as either stem or spring donors. The Ocean Blues as well, they have a bump that starts right up-top, so some fast-curve (or fast-curve progressive) springs in there might accomplish your goal.
If you can find them, another switch that might be a good candidate to swap a fast spring into is the TTC Matrix-01 Tactile. (An easier-to-find alternative would be the Bluish White which uses the same stem - but to get full-travel from it you’ll have to remove a small silicone plug from the center tube.)
There’s also the Ink Kangaroo, though it does have a shorter travel. Bump is right at the top, though, almost “T” shaped as opposed to “P”.
All three of those switches have a pretty similar character and check at least a few of those boxes; I think a fast spring in any one of them has a good chance of satisfying:
Ink Kagaroo, CS Jelly Blue, Matrix-01 Tactile - all clean top-bump tactiles.