Your sciatica has little if anything to do with your saddle. It's the very process of riding that you have you have to deal with.

As you ride, your alternately stressing your back side to side with every pedal stroke. The higher the gear or the stronger your legs are, the greater the loads. I've been dealing with sciatica for almost 20 years now, and the key is management. Strangely enough, I can ride pretty much OK, but can walk more than a mile or so before the pain comes on.

Here are some things that may help--

Having someone ride behind you and watching if your hips rock as you ride. If so, lower your saddle 1/4" (or do so on a hunch), and see if things improve
Look for a slightly wider saddle, and sit so you're well supported on the wider area to stabilize your hips. (takes some dialing in).
Doing various stretches and moves to loosen the part of the spine where the sciatica originates. Also do stretches to try to lengthen your hamstrings. Do some of the hamstring stretches before and after riding.
Experimant with bar height, and hand positions to see if any riding position is better, or just mix things up as you ride.
As pointed out above, stand periodically or move otherwise to keep things loose and break up muscle spasms before they start.