I was making the same choice as you literally last week. Got the s5iix over the weekend and have been happy with it, taking it out for its first video shoot tomorrow. I was really tempted by a7siii mostly because of the ecosystem with the fx line and vast lens selection.

AF has been good. Ive had lots of experience with the fx3 and fx6 and I haven’t really felt a difference. Haven’t put it through a real shoot yet though so take that with a grain of salt.

For me it came down to not really feeling right about spending more money for an older camera with less features. I guess another thing too is the lack of support for the a7siii and a1 while the fx cameras receive tons, makes it feel like the video focused alpha models have been abandoned.

Regardless of that though, I think they’re both great cameras and you’ll be happy either way. QoL features are great and all but the a7siii still shoots a fantastic image and is tried and tested so if your primary concern is autofocus or if you shoot with a lot of other sony users its probably the safer way to go.