I actually got a used A7s in near-mint condition a couple of weeks ago, but not for video; I am using it for stills only. I currently use an A7iii (also only for stills, I don't shoot video with Sony) and the A7s feels like a step backward in many ways but it's smaller and lighter, which I like. I heard from multiple photographers that there's something special about the combination of the A7s and vintage lenses (which is almost all I shoot with), but I'm not seeing it; I've used all my lenses on the A7s now and honestly can't see any differences between it and what I'm getting from my A7iii. The files are lower resolution and smaller, of course, but in terms of image quality, rendering, etc., they seem identical to the A7iii. To do a controlled test would require setting up a tripod and changing out the cameras using the same lens shooting the same subject in the same light, but if there are differences they must be pretty subtle.
The A7s is less customizable, has an even more primitive menu system than the A7iii, has no IBIS (which isn't an issue for me but would be for some people), and the battery life is about half that of the A7iii even with higher-capacity third-party batteries from Wasabi or Neewer. The EVF is worse than that in the A7iii and the refresh rate is noticeably slower. I see jello if I move the camera while looking through the EVF.
It's a good camera and I'll keep it, but I probably would have been happier with an A7c or another A7iii as my second camera.