Speaking strictly of value, the A7SIII is likely your best choice if you want to do both photography and video. It's a low-light beast and it's great for both. The FX30 is also a good choice if you really only want to shoot video though. You can't go wrong either way 🙂
as someone who does both photo and video, the A7sIII all the way!
The better comparison would be the FX3 to the A7SIII (which it's pretty much the same guts so a refurbished a7sIII would treat you right). the F30 is GREAT, but it can't handle low light at 12800 CLEANLY like the sIII can.... that alone is worth double the price of the FX30
I think the a7SIII is better as you can get FF capability and much lower light captures with better quality and lower noise levels. If you are constantly using mics that require phantom power, then FX-30 or FX-3 would be better but otherwise, I'd go with a7SIII.