I presume you are aware that the A9 and A7III are full frame cameras and the RX10IV has a much smaller 1" sensor: so it does not have the A9 sensor, it has the A9 processor. I own an A9 and an A7III and would only recommend the A9 if you really do need the incredibly fast AF it offers. Otherwise, the A7III is a brilliant general purpose camera. I use mine for macro work and my A9 for birds and insects in flight.

I use a Neewer 2.4Gram Wireless 1/8000 HSS TTL LCD Display Master Slave Flash Speedlite on my Sony A9 and A7III and do not consider it to too big. It has a guide number of 60 and has an HSF function. It is si much cheaper than a Nikon flash and works well. Prior to that, I owned a Metz AF64 Digital which I used with my Olympus EM1 mark II. Again, no problems size wise. Although the Metz was quite expensive it was still cheaper than a Nikon flash. I also used a Neewer on my D500. The Neewer and Metz are about the same size.

I was recently looking at a second hand copy of a Sigma 150 - 600mm for my Sony A9/A7III, and following some YouTube research it appears there is little difference optically between the contemporary and the sport. I was a little surprised, given the difference in price. The sport version does have a better build quality and weatherproofing and the hood is metal. But, if you do not intend shooting in pouring rain, the price and weight difference may make a contemporary version a better purchase. Having said all that, when I owned a Nikon D500 I chose the Nikkor 200 - 500mm f5.6 over the Sigma/Tamron 150 - 600mm lenses. The Nikkor is a stunning lens and readily available secondhand now. Though I owned a Nikkor TC14EIII I cannot recommend using one with the 200 - 500mm f5.6. I always felt it was a bit soft and only used it on static subjects. It worked brilliantly on my Nikkor 300mm f4 PF ED VR, though.

MPB is a company I have used and would happily use again for a used lens. They always provide a detailed description. You might consider the Nikkor 200 - 500mm f5.6. It is a truly brilliant lens. I had one on the end of my D500.

Check out an Olympus EM1 mark II, the IBIS + lens stabilisation is outstanding. They are readily available on the secondhand market. If you can stretch to it, check out the Olympus EM1 X, apparently it is even better. I owned an EM1.2 + Oly 300mm f4 + 1.4TC and it was brilliant for birds in flight. I preferred it to my Nikon D500.

It isn't a fix for anything: it provides continuous EyeAF (though it may be switched off) and Animal EyeAF. If you own either a Sony A7RIII or A7III you might consider accessing the Sony website and viewing the updates in detail. You can then assess if they are of value to you. From my point of view, the A7III and A9 updates have been of great value to me.

Because I shoot out in the wild and have to fight my way through brambles, stinging nettles and thistles, I favour a 180mm macro lens. Until a few months ago I owned a Sigma 180mm f2.8 OS macro lens and used it with a Nikon D500. It is a damned heavy lens though, and not really suited to being handheld at low shutter speeds. My Nikon/Olympus EM1 mark II kit became too heavy in the end and I now use much lighter Sony gear. I currently use a Sony 90mm f2.8 macro on my A7III and a Sony 100 - 400mm G Master + Sony 2xTC/auto extension tube for insects which are a bit too far away for my 90mm macro. Works for me.

My experience with MPB is that they are very good.

Take a look in your local (or on line) angling shop. I regularly see anglers with the type of product you are describing.

I own a Sony A9 and Sony A7III and belong to the relevant FB groups. If you own either camera, but do not belong to any Facebook groups for those cameras, I am happy to ask the question for you, if you wish, and post the answer.

It is a brilliant lens and I had no trouble with mine on the end of my D500 - but that camera is pretty good in lower light/higher ISO. I also owned a Nikkor 300mm f4 PF ED VR and used it with a Nikkor 1.4TC when required. Though more expensive brand new, you may be able to pick up a good used version. It is incredibly lightweight and produces excellent results. I did, however, favour my 200 - 500mm.

This is a question I posed myself sometime ago. When I owned a Nikon D500 system and Olympus EM1/EM1 mark II system in tendem, I owned the following lenses for my wildlife photography;

Nikkor 300mm f4 PD EF VR + Nikkor TC14EIII
Nikkor 200 - 500mm f5.6
Sigma 180mm f2.8 OS macro + 1.4TC
Olympus 12 - 40mm f2.8
Olympus 40 - 150mm f2.8 + 1.4TC
Olympus 12 - 100mm f4
Olympus 300mm f4

Today I own a Sony A9 and a Sony A7III and have the following lenses:

Sony 28 - 70mm f3.5/f5.6
Sony 90mm f2.8 macro
Sony 100 - 400mm f4.5/f5.6 G Master
Sony 1.4 TC ( to be purchased soon - ish).

The above three lenses (and the 1.4 TC) allow me to capture virtually everything I wish. I have no need for any more.

I have used two camera bodies in tandem for years for my wildlife photography. They have not always been the same format; up until recently I owned a Nikon D500 and an Olympus EM1 mark II. In the winter months I would have a 200 - 500mm f5.6 on my Nikon and a 300mm f4 on my Oly. The rest of the year I would have a Sigma 180mm f2.8 OS macro on the Nikon and the 300mm on the Oly. I now have a Sony A9 with 100 - 400mm permanently attached and an A7III with a 90mm f2.8 macro.

Yes, I did see it; thank you. I had hoped someone else already had experience of using the A6400 for BIF/insects in flight. As it happened, I found a mint condition A9 at a reputable camera shop and purchased it. Problem solved.