Traditions makes a gun called a pa pellet ultralight that has a 1:28 twist and can from videos I’ve seen can shoot black powder subs. The biggest reason for wanting to use a sub is due to the corrosive properties of black powder. However it seems that traditions is not held in high regard. Also the tc firestorm, although not in production also seemed to be a good “modern” flintlock. So I feel like it definitely does exist, just is not plentiful by any means.
I have no experience with flintlocks, or Traditions quality either, but thought I'd address your comment in bold.
There can be a lot of misconceptions about real black powder (BP) and how corrosive it is vs. the subs claiming to be "non-corrosive". An example is when Pyrodex first came out, many read that "non-corrosive" part and thought they'd be able to shoot it and not clean right away.
Ask anyone that did that and then left their gun dirty for a few weeks and they'll tell you about how badly it corroded their barrel.
It was basically a play on words by Pyrodex, as they meant it is non-corrosive BEFORE BEING FIRED. Once fired it is just as corrosive as real BP, some will argue even more so. Real BP is also "non-corrosive" before it is fired, it doesn't cause any issues sitting in a barrel for a while without being burnt.
So, other than using BH209, which won't work in a flinter, you will need to clean whatever weapon you choose fairly soon no matter what real BP or sub you use. I personally was raised to clean after using any kind of rifle, it is just part of the process if I go shooting.
I have read that triple 7 is pretty nice for light fouling, and even work with a guy that is very gun knowledgeable that will leave his caplock uncleaned for a couple days during hunting season using Triple 7. His rifle shoots best with a fouled bore and he says he's had zero issues doing so. This guy eats, sleeps, and you-know-what's anything firearms related. He isn't a blow-hard, he really does know his stuff.
Real BP is not some terrible, scary, ruin your barrel powder in comparison to a majority of the subs though. I personally would choose 3f real BP for the main charge and 4f real BP for the pan in a flintlock from what I've read. Be aware that 3f is finer powder and thus creates more pressure when going off compared to 2f, do not exceed max charge recommendations of the manufacturer of whatever rifle you end up getting. The manual for it should have both a 2f and 3f max charge listed in it.
As far as a lighter charge, many deer have been ethically taken with charges in the 60-70 grain range. What charge you settle on will partially be determined by your rifle. For example, it may shoot poor groups at 60 grains but then really tighten up at 70. You'll figure that out with some range time.
The heavier the projectile you choose, the more it's going to smack your shoulder. Since you are wanting a lighter weapon that is easier to hold up when aiming, that's also going to equal more felt recoil.
If there are any muzzleloader groups or clubs near you, they will likely be more than willing to help you and even let you test shoot some of the rifles they own I'll bet. This would be a wealth of knowledge for you. Many are on social media such as FB.
They have a “Snappy” 1:24 Twist, Specifically designed to Shoot Bullets. Since you want to Shoot Bullets, i would NOT get Anything with a Slower Twist Rate than 1-48
