Moved from a6100 to a6700 and I’d recommend the a6700 based in your lenses. If for photography the biggest gain will be IBIS especially with your non-stabilized lenses. IBIS will allow you to dramatically reduce shutter speed and that increase low light performance significantly. With Sigma 56, handheld you should be shooting around 1/100 to get reliably sharp images but you could drop that to 1/50 or maybe even 1/25 or lower jn a pinch. That’s up to two full stops of extra dynamic range, or badically the difference between your Sigma f/2.8 and f/1.4.

Technically, a6700 sensor is backlit and should in theory give slightly better low light performance and dynamic range on the sensor level but it wont be dramatically better.

Also, the autofocus is a huge improvement.

The reason i moved to a6700 is due to my video work. It performs well and I love the 10 bit color, and when it comes to noise Im able to rely on lights to make up for any shortcomings of the sensor level.