I am upgrading to the a7S III eight months after purchasing a Sony a6400. The a6400 was a great beginner camera to get me back into the basics. To get reacquainted with photography and videography. However, its limitations really started to show after a few months of usage,
The a6400 does not have in-body image stabilization. You can mitigate some of the shake using lenses that have built-in image stabilization, however, that pretty much limits you to Sony-branded lenses. There's no beating a camera and gimbal set up, yet I find carrying a gimbal very cumbersome. Eventually, I stopped using my DJI Ronin-SC gimbal altogether because it was a pain to carry around. With the a7S III, there is built-in gyro data for post-image stabilization. Still not as great as carrying a gimbal but damn near close.
The other issue was that the a6400 is an APS-C crop sensor. I honestly didn't think I needed a full-frame camera, however shooting in 4k, the a6400 has a slight crop in on the mage. That's two crops. One because of the sensor size and the second when recording in 4k. I use a versatile 18-105mm lens for almost everything. When I vlog and have the camera pointed at me with my arm extended, the framing of my face was too close.
Low light was another huge factor when choosing the a7S III. The a7S III is an absolute beast in low-light conditions. Considering that I barely want to carry around a gimbal, imagine how I feel about a lighting setup. This is a huge plus for vloggers.
With the a7S III comes a new flip-out screen that is perfect for vlogging. A huge improvement over the a6400 and its tilt screen. The flip-out screen feels more reliable whereas the tilt screen seemed like it could be a major failure point.
If you plan to run a mic on the a6400, you'll need a mic shoe relocator or an external monitor. You cannot use both in stock configuration due to the placement of the shoe and the flip-out of the screen.
I also like that I can have the back of the a7SIII screen closed when the camera is not in use whereas the a6400's screen is always exposed.
The downside of upgrading to the a7S III is that the new setup is a bit heavier and slightly bigger than my a6400 rig. The a6400 can probably pass as a point and shoot with the kit lens. Thus allowing me to use it in events where professional cameras are prohibited. With the a7S III, that'll be much trickier. or perhaps impossible.
The 12MP sensor is fine for video, however, it's on the low side for images. I can't do massive prints from images taken with a7S III, however, it should serve perfectly fine for social media. Just be sure to frame your shot perfectly because there won't be much room for editing in post.
The extensive E-mount line of lenses meant that I could continue to use my existing Sony lenses. Unfortunately for me, I did not plan ahead and buy FE lenses. Instead, my collection of E-mount Sony lenses were for the APS-C crop sensor. These lenses will still work, however, I would need to zoom in when recording to eliminate the black circular vignetting around the edges. For this reason, I went with a 24-105 lens for my new setup.
I also had to upgrade my memory cards. The previous SD cards that I used on my a6400 were too slow for the a7S III. I went with the highly rated ProGrade v60 line SD cards. It's a bargain price for its size and performance. Additional purchases also include a SmallRig camera chassis to mount peripherals onto my camera. And just like my a6400, I went to RAVpower for extra batteries and a double battery external charger.
The a7S3 is very expensive for amateur use. I'm treating this as a hobby with the potential of making additional income. I'm also going to sell my a6400 with all of its accessories. Cameras hold decent value so if I change my mind, I can always sell everything and move on.
Upgrading to the a7S III wasn't a hard decision to make if money wasn't an issue. Decide on what features are most important when buying a camera so that you don't need to upgrade as fast as I did.