Slugoon

Sounds like you are selecting 'Film Simulation' from the main menu instead of 'Select Custom Setting' which is another option further down. The latter is the same as the one you are successfully using in the Q menu. The former will only change the film sim. Why not just use the Q menu?

Good advice from Herco. Why do you no longer have or want the D810 if you owned one? That might also offer a clue as to what will suit you best.

Compared to the D810 you're going to lose a bit of spatial resolution and around a stop of dynamic range. With regard to spatial resolution, science would suggest that 24/26 megapixels is more than enough for anything unless you are severely cropping or printing large and pushing your nose up close. I rarely print larger than 8 x 10 for which I don't even need 9 megapixels. With regard to dynamic range, well, you get a stop less. Bear in mind though for landscapes you're probably going to shoot at base ISO most of the time so does it really matter? The masters coped with 5 stops of Velvia dynamic range and inferior lenses. Current APS-C is over 10 stops. These cameras are more than capable of stunning, world class images. If you can't get one, it's not the camera! Picasso used basic house paint. This guy used an X-T2 and X100F: https://www.uklandscapephotographer.com/fujifilm-x-in-the-faroe-islands/

I have never used the XF10-24 as it's way too wide for me and I wouldn't have the skill to use it! Neither would I bother with a prime lens for landscapes (purely personal opinion) due to the sheer compositional inflexibility. You'll end up wanting other focal lengths and changing lenses is a pain and even inadvisable in inclement weather plus quite often a small turn of the focus ring is handy to get rid of an annoying element on the edge of the frame to save cropping later. If you're the sort of person who sticks their nose two inches from a 36" x 24" print or scrutinises images at 100% on a screen then maybe a prime lens might give you some extra sharpness but in the real world I don't think the trade off is worth it. A lot of time you'll be shooting closed down where the primes start to lose their advantage. I have the XF16-55 and it is outstanding at every aperture and focal length and covers many of the focal lengths you need in a day's shooting. It is also waterproof but it's heavy and has no VR. I use it with an X-T3, battery grip and tripod over the Peak District so the weight and lack of VR is not a problem and it feels very balanced with the grip plus you get the two extra batteries to shoot all day. If your primary use is landscapes and seascapes then my recommendation for the ultimate Fuji rig would be an X-T3, grip, XF16-55 and XF50-140, or the XF10-24 if you love the really wide stuff. I would also seriously, seriously consider finding a nice X-T2 instead of the X-T3. I've owned both and the IQ is every bit as good if you don't need the extra features of the X-T3. With the X-T cameras you also have the option of stripping it back to just the body and a small prime like the 23mm or 35mm or even the ultra-light XC16-50 zoom, which is what I use a lot of the time instead of the XF16-55. This makes it a compact and lightweight combination for casual days out. Let us know what you decide!

Excluding a couple of entry-level and smaller-sensor models, your choice will come down to one of the following series: X100, X-Pro, X-T, X-H and X-E. All of them from the past few years have 24/26 megapixel X-Trans III/IV sensors and there is very little to separate any of them in terms of raw image quality, they are all capable of spectacular images. I'd say there are two things for you to consider. One, is that for landscapes and seascapes the lens you choose is just as important as the body and may even dictate which body you need as some of the larger lenses just don't balance well on the smaller bodies. Are you thinking of the larger zoom lenses or the small primes for your landscapes? Two, what else are you using the camera for? Is it solely landscapes or do you need versatility for other uses? As well as lens choice, a lot will depend on personal preference of how each feels in your hand and which has the features you need or want.

I can't speak for the X-T4 but I currently have an X-T3 and X100V and have previously owned an X-T30, X-T2, X100F and X100T (that's three generations of Fuji sensors). I shoot with my cameras set to RAW but then use X Raw Studio to 'develop' the images I want to keep so although I am technically using camera JPEGs I have the flexibility to adjust exposure, WB, film sim etc. before I process the JPEG. For me, I enjoy the constraints of a limited set of adjustments and there's a magic about the camera JPEGs that I just can't reproduce on the many desktop raw converters I've tried. The JPEGs are incredible but may not offer you the scope of adjustment you require. With regard to visual differences there's definitely something different between the current X-Trans 4 and previous X-Trans 3. The colours are slightly different and the shadows aren't quite as black to my eye. But it's not 'better', as such. I still find the X-Trans 2 images captivating (X100T) but there is more smearing and less detail at higher ISOs. If you don't need some of the more modern features, I'd honestly get a nice X-T2 and spend the rest on good lenses. The images are as good as the X-T3 in my view and I doubt the X-T4 will look any different.

Just to provide a balance of views, I haven't missed the D-pad at all and yet I've also disabled all touchscreen functions, which to me feel gimmicky and unsuited to a camera like this. Fujifilm has to walk a very thin line in balancing the wants and needs of so many photographers and I think they do a good job overall. If you're regularly changing white balance, and film sims etc. on the fly then I can understand why the D-pad is important and a touchscreen replacement wouldn't cut it for me either. I was very sceptical about the flip screen but it's so well designed that you don't even know it's there when stowed and I've actually found myself using it on several occasions already. It's a fantastic addition! For anybody unsure whether to 'upgrade' from a 100F I would definitely evaluate your needs before spending lots of money. The image quality and autofocus speeds aren't any better in my opinion, the extra pixels don't make any practical difference and things like video capability and weather sealing may be irrelevant for many people. And yet, the new version is strikingly beautiful, the lens is tangibly better and the extra JPEG processing options (for people like me who don't use third-party RAW converters) I now couldn't be without. The 'F' can still take world class images since that is mostly down to the photographer. The 'V' has some nice additional features that may or may not matter to you but I bought mine in a heartbeat.

When you physically select a shutter speed it can be fine tuned up or down 1/3 or 2/3 EV with the rear command dial. Is it possible that you accidentally knocked it down to 160?

Yes, on the X-T2 the function button would bring up a temporary menu where you'd select which variant of face detection you wanted. The X-T3 frustratingly doesn't do this. Instead each press of the button just toggles it on or off in the background with nothing to tell you which way you switched or which variant you'd like. That has to be preselected in the menu beforehand. I really hope Fujifilm can revert to the X-T2's way of doing it as that was much more user friendly. Note that if you assign 'Face Selection' to a Fn button instead of 'Face Detection' it at least flashes a banner on the screen to say whether it has turned on or off.