I was toying with similar recently - have piles of FE gear and an A9m1, mostly do wildlife, so like children, fast focus, good tracking, and good burst, sans blackout if possible, are critical.
Got all the OM gear, including the OM-1, 100-400, 1.4x TC, kit 12-24 … played with it for a day or two, copied the images over to my laptop… and was floored by how bad the shots are compared to FF! I’ve been using FF for 12 years or so, so I think I’m just spoiled … every time I try something else (including an LX100m2 I picked up to try out as a travel camera), the IQ just kills me. Even running it through Topaz, it just doesn’t come out as sharp or crisp, with practically no room for cropping with the low MP count on the sensor (not that the A9 is much better in that regard).
I’ll have to go MFT one day, like so many people in my camera club … you get to an age where FF and that big heavy glass is just too much… but until I have to, my goto will always be the FF.
Yeah, I sent my A9 off for surgery - https://www.lifepixel.com/photography-gear/anti-aliasing-low-pass-filter-removal 😄
I think my initial “yuck” with the OM-1 was the 12-24 just taking bright landscape shots … everything looked, muddy. Of course, a kit lens on a Sony wouldn’t really shine either.
To be fair I did only give it a few days and then gave the whole lot of it to my dad who was shooting with my old FZ-1000.
And yes, with my 200-600G, monopod is a minimum, tripod if I know I’ll be stationary. Handheld is practically never, but I have in a pinch and with sufficient bracing, and for just one subject (before it flies or runs away typically), I can manage. And I’m only 44 😄 … hoping I can keep using it for another 10 years or so at least!
I only use that for wildlife though … for the odd recital or what have you the boy has, I bring the 70-200G with the 2x TC.
100% agreed about “the one you’re carrying”, which is why I haven’t actually put much money into anything lately - I do about 90% on my Xiaomi 13 Ultra, with a 1" sensor like was in my FZ-1000. I find I only use my big expensive gear when I actually go out to “do photography”, which is pretty rare these days. My son is 14 now too, so there’s way less recitals and games and events and such for me to shoot.
Groggy day today here for me, so a slightly rambley and discombobulated response, but, as I’ve typed it, I’d might as well post it now :-)