given your backpacking hiking experience, that is really good that you know what light setups work for you for given temps. Being on bike, even with a light setup allows for more leeway of having a bit more clothing and space for adding in store food purchases at end of day--thats why even with a minimalist setup, its nice to have some spare room to temporarily add in groceries.
I mentioned four panniers because you asked about a fork to use a lowrider on, so thought you wanted to go that route.
Your bike with a 34 chainring and 42 cassette, along with a guess of 700x50 tires, gives a gear inch range of gearing from 23-88 gear inches. This "gear inch" term is still a good standard to look at, as you can see in black and white what your gearing is. Here is a chart with your bikes setup. I've toured a lot with varying loads, and 23 is still too high for me, its nice to get a bit lower, but having a 1x drivetrain, you are always limited in compromises.
Your 88 g.i. top gear will spin out roughly at the speed you mentioned, my touring bike has a range of about 17-103 g.i and I spin out at around 50kph, 30 mph, which is fine by me, but with a 1x, you don't really have an option if you want to lower your gearing a bit, unless you go with a bigger cassette, which will have slightly bigger jumps between shifts.
2x setups really do have the advantage here to have a wider range of gearing, but you've got what you got.
https://www.gear-calculator.com/?GR=...&DV=gearInches
there are lots of smaller rear panniers available, Ortlieb even makes bikepacking rears that combined are about 25 liters, and there are numerous other brands out there also, especially with the small rear panniers becoming popular again in bikepacking.
Your frame appears to have ample room for a partial frame bag that could still allow water bottle access, again--tons of options out there you can look into. I have a couple of partial frame bags, one shorter that easily allows for one full size vertical water bottle and a slanted shorter one. The other partial frame bag is bigger and fits all the way under the toptube, limiting water bottles access, although your frame has more real estate than my frame.
the front flat rack option is also there for you as you say, and you could use the same fork.
You've got lots of options to think about, and as you say, you can choose things that work well and have a nice aesthetic also.
I'm running 40mm wide, so maybe just a little better. I'm not sure how to work that calculator, as I've seen that before. However, I'd guess that the 23 gear inches you cite would be maybe 22. Would I like a little lower, yes just in case.
I'm sorting out what I'd like the bike to look like and how I want it to feel while riding. Ortlieb Sport Packers look appealing, as does a Carradice style seat bag. I've had a Dill Pickle in the past, but that particular one needed a little more support (it got a bit too loose for my tastes).
I know its a contradiction from what I wrote earlier, but I'm even looking at a setup for large saddle bag, large bag in front (resting on a small front rack), and a frame bag like you mentioned. Maybe front low riders and saddle bag. The weight is a bit higher with the non low-rider setup, but I like the way the bike feels with the weight towards the center-line. Panniers are nice and low, but when I stand to pedal (up steeps for example) I don't have the same feel/rhythm that I like while riding a bicycle. Have you noticed that?
I'm having fun with it though. We are fortunate to have so many options. Also, I'm happy that I've tested so much and found that I'm very happy with less. I was looking at my 16 pound kit played out one morning in the Rockies and saw a couple of things I could do without. After getting caught in a hail and rain storm at 11,000 feet though, I thought an emergency set of warm clothes would be smart. I enjoy traveling less when I have more to organize and keep track of. Got back early August from a week in the north Georgia mountains, and if it weren't for multiple saws and support gear (I hike and volunteer to clear trails for the TNGA), I'd have only one daypack full of my things (drove my Jeep to the area).
BTW - The guy I'm working with at Spork Forks is excellent - very helpful. I have a couple of suggestions for their website, as the explanations of their products are hard to follow. However, when I speak to them, all is clear though.