I have some experiences about M4/3 system and also cross platforms that I use at the same time:
I have used olympus EM1iii with panasonic 100-400mm (eqv 200-800) among other lens for several years as a lightweight setup. Although it seems to work fine, there are a few things that I hope could be better.
1. The depth of field is not great, eqv f12.6 at f6.3. It is very difficult to control the background if I cannot move around much. It works fine for open space where I can move around and choose better background.
2. Higher noise, I get struck to mostly around iso 800 all the time. 1600 is possible but is much noisier. Topaz and the like helps cleaning it up a bit.
3. Poor AF for EM1iii in low light. I think it hunts at iso around 2000 with f6.3. I am not sure how the OM1 AF performance in low light is. For me, low light AF performance is as important as bird detection and fast tracking AF.
I have been wanting to try the 300f4 pro but I cannot find the used ones here. $3000 new is a bit too much just for trying. We cannot rent it here. 150-400 pro is not available here either. I was gonna jump in to place and order but afraid that I might have to camp out after that if the boss at home knows about it. I think if OM does come out with the 40-250 f2.8 with reasonable price it will be really awesome as we can us it with 1.4x and give us evq 700mm at f4 (eqv f5.6).
I also have a sony A7iv and a6600 that I use with FE200-600. So most of the time I take both system, use A7iv/200-600 as the main kit on tripod and EM1iii/100-400 as a quick second setup for a quick shooting off a tripod. In early morning/late afternoon or in the woods where the light is low, besides the poor AF, I feel that the image quality of the EM1iii, when the light is poor is worse that A7iv even after cropping around 10-20% of the A7iv to have the same eqv 20MP. A6600 (eqv 300-900mm with 200-600 lens) is ok but for some reasons I never like it. I use it when I need very long reach. I think a7iv/200-600 plus 1.4xTC give me the best result but then it is f9.
Then I get the A6700 to replace A6600 just to see how good the mini A7rV is and to benefit 4k120P video. I hope to use this as a permanent 300-900mm setup. Although AF is very good, I never like it, maybe because of the small body, everything is cramped into small form factor, evf, button, etc, and it is much less ergonomic. Using a6700 with 200-600 handhold is tired (and hurts my palm) much quicker compared to a7iv and M4/3. I put a smallrig bottom plate to make the body easier to grip on but it is still not very comfortable at all. Noise performance is good. I can get very good images with noise reduction for most iso less than 3200 now.
What comes at surprises is the Canon R7 with ef100-400 f5.6 is ii. It gives eqv FL as 160-640mm. This one turns out to be the setup I like most. Very good bird AF, light enough and can crop a bit more to have eqv 800mm at 20 MP. Noise is much better than EM1iii at iso 1600. I also put on 1.4x and the result is very good. The only complain I have is that it is getting heavier and longer when adding EF/RF adapter and a 1.4x TC so it does not have a weight/size saving anymore. RF100-500 is too expensive and I am not sure if f7.1 is ok for my use or not. So I trade them in for Z800PF and Z9.
So now I sold all canon, and plan to sell most of sony stuffs but still having a full set of M4/3 (14-400mm) Still not sure if I should keep them or sell them off. I have been use it as a “better than bringing nothing” system for a trip that I cannot take a big setup. It has been giving me many nice photos, even for birds. I have been waiting for indepth comparison between the OM1 and the new Pana G9ii and may decide later. If I can afford it, I definitely will try OM1/G9ii with the 150-400f4.5 and will keep it for lighter setup. My lighter setup now will be Zf/Z8/Z6iii/Z7iii with Z24-105f4 and Z180-600 (plus Z14-30f4).