I know about this issue but i start using ES becasue I thought that X-H2S is designed specially for shooting moving objects as it has sensor with the fastes readout on a market and drive modes 30fps and 40fps avalible only in ES. And in ES you doesn't have blackout, freezing or low frame rate in evf during  continous shooting. But i'll try do few test to chceck 30 and 40 fps and also MS, becase to date I used only 15 and 20fps. Me friend who shoot also airplanes with Canon R5 use ES and had problems but related to the IS which was fixed by updating lens firmware. That's why i strated suspecting that may OIS can do bad job during handheld paning. I'm guessing what for example do the subject tracking. I found that after turn it on, you can't change metering mode, can't change limiter in camera, and maybe there are more settings blocked, maybe it changes how OIS work during paning?

I have a pair of X-T30-II's as they are light and dependable. I mainly use fixed focal length lenses, the more compact Fujifilm ones. However, for longer reach I have been using the 100-400 Fujifilm zoom. Yes, it is not the lightest but it is quite well made and works well for me, even though I don't like zooms very much. I picked mine up from a reputable e-bay seller,, and it works fine. The lens stabilization works well even though the body doesn't have IBIS. I'm not entirely sure what the "Pro" designation on some of the lenses Fujifilm makes as plenty of my compact fixed focal lengths don't have it on them and they are superb pieces of glass. Frankly, if it works for you I'd just ignore the "marketing" labels. Now 400 mm doesn't seem like it has that much reach, but remember this will be the equivalent of 600 mm for a full frame digital or 35 mm film camera. 10 years ago, unless you were a professional photographer hiring obscenely expensive lenses, 600 mm was a very long lens for anyone to own. That is a lot of reach. Anything more than that is going to be substantially larger, probably heavier and with less light transmission, oh and very much more expensive. I like the 100-400 mm a lot, especially for the money, and it gives me very reasonable results. I'd take mine with me on a safari any day. I think you will be able to keep far enough away from the big kitties to stay safe. I use mine with a mono-pod quite often, and with the lens stabilization is turned off.

Ah, okay, you are panning while hand-holding and not while using a panning tripod. You mention you are using the electronic shutter. That may cause you a bit of trouble: https://fujifilm-dsc.com/en-int/manual/x-h2s/menu_shooting/shooting_setting/ scroll down to the Shutter Type section for the notes on shooting moving subjects. I have heard that increasing the fps does make the focusing sensor work better, because it takes more samples, but I do not know for certain one way or the other. Just to cover all bases, have you considered using a smaller aperture (higher f-stop number)? Even with zone focusing, too small of a f-stop number will still blur things outside the immediate focus area, which can be tricky to center on a moving object. Using the idea of shutter speeds twice the focal length should still work, that is what folks did before IS, OIS and IBIS came along.😀

I'm tracking airplanes, taking photos during paning . I use AF-C, ES and 15 or 20FPS, subject tracking off, center point or center zone. In static shots OIS is very efficent because I can take sharp photos on 140mm and 1/10s but with paning maybe the best option is to keep up shutter times 2xfocal lenght. Today i took a series of photos on the times close to focal lenght. Many of those was shaked only in small areas. For example plane was sharp but nose or winglet or end of tail was slightly shaked. Maybe i have to use 30 or 40 fps?

To answer your question, if you are using a Fujifilm lens with OIS, turning off the OIS switch on the lens will also turn off IBIS. If you are using a lens without OIS, you will be able to turn IBIS on and off in the menu: Shooting Setting > IS Mode. What you are describing sounds odd though, are you trying to take a shot while panning or are you moving the camera and then waiting a bit for things to settle down and then taking the shot? Nowadays some folks claim that you do not need to turn off the image stabilizers when you are using tripods, even going so far as to say that if the tripod is low grade, then it will still transmit small vibrations which you can avoid by having IBIS / OIS turned on. Yet others say it is still best to turn the is stuff off while using tripods. p.s. welcome to the forum.