Yes the OM-1 has a Sony-made 20mp stacked sensor, but the stacked sensor in the Sony RX10 IV has a top shutter speed of 1/32,000 sec, and can do an amazing 960fps super-slow-motion video capture. The OM-1 has a top shutter speed of 1/8000 sec, and has a top video frame rate of 60fps.

There is a difference.

I love that my Sony A9 and A1 also have a 1/32,000 second shutter speed. That allows me to shoot in bright daylight and still use a wide aperture to get that shallow depth-of-field look, without needing to use a ND filter.

Here a shot of friend Brooke. H., in bright sun in Scottsdale, Arizona, USA. Sony A9, 1/32000 sec shutter speed, Sony 135mm f1.8 lens, at f1.8.

Cheers and best to you

I was north of Svalbard on a ship when I saw some professionals getting better pictures than I with a much smaller rig. I was using full frame Canon, they were using Olympus. I've since sold all my (expensive) Canon gear and converted completely to Olympus. I've got the EM-1.3 and the OM Systems OM-1. I've also the BWL, the 300mm f4, the 12-100, the 1.4x and 2.0x extenders, and assorted other lenses that aren't relevant to your question and style of photography. I've had the Panasonic 100-400mm but I got rid of it (too soft) and I know from other people's experience that the Olympus 100-400 is better than the Panasonic, but it too suffers at long ranges. Based on my experience, you might consider a used EM-1.3 if you want to keep total costs down. There are quite a few of them out there (mine included shortly) for sale because people (like me) have moved up to the OM-1. I love the OM-1...especially for BIF. The AI subject detection is fabulous for birds, and when you use any Oly lightweight lens you can slew the camera rig around hand held for very fast action....particularly when shooting from ships. My recommendation is get used stuff to keep inside your budget. But, get the OM-1 if you can afford it after getting the 300 f4 and the 12-100 PRO. Add the 1.4x for some extension on the 300, but don't go for the 2.0x. The 12-100 will be on your camera all the time (except when you are doing wildlife/birds). You'll really like the OM-1 performance (if you can get it inside your budget)...but the EM-1.3 is pretty darned good too. No question you definitely want the 12-100 and 300f4. Don't skimp on the glass. You'll definitely be able to see an IQ degradation if you choose something else....Other lenses are good (don't misunderstand me), but these two are noteworthy. I know that lots of people demand zoom at long ranges, I like it too (that is why I have the BWL), but that is out of your price range and the 300 f4 is going to stretch your budget as it is....but you said you pay close attention to IQ. Therefore I'd suggest you definitely get the 300 f4. It (with the 1.4x on occasion) will keep you satisfied. Later, when the boss gives you permission, go for the BWL....but you'll need to be very very good to the boss... LOL.

If going smaller is very important for you, then Yes go OM-1 system and micro 4/3rds over the 1 inch sensor in your Sony RX10 IV. You will lose the advantages of that stacked sensor in the SonyRX10 Iv, but that might not matter that much to you.

Larger sensors do generally have advantages in low light capabilities, and shallower depth of field when you want that effect.

The OM-1 system is well thought out and offers an excellent selection of lenses.

I personally have shot with mirrorless micro 4/3rds, APS-C and fullframe cameras . And for my professional and personal work I prefer fullframe mirrorless. I wont go into all the reasons but simply I love ultimate image quality, resolution and detail.

But I will not try to persuade you to go fullframe, as that would be much heavier and bigger than either your current Sony RX10 IV or an OM-1 micro 4/3rds system.

I think for you the OM-1 system is a better fit that ANY brand APS-C or fullframe system.

Cheers and best to you.