I have a Chevy Astro and am on an Astro specific forum.
Another member there is making swivel seat bases that will retain the factory height, which many solutions raise the height.
Good if you're of average to shorter height, not so good if you're tall.
Ideally you'd want to find some from another Chevy/GMC van so in theory everything bolts up and no seat adapters are needed.
Also I've seen a lot of buzzwords flying around about swivel seats like DOT and approved and I want to say I've seen not recommended for driver's seats.
Some conversion vans came with them so you may be able to find them there.
Any who, I want to get one for my passenger seat to flip around for sort of a recliner/reading nook.
I'll ask the guy making the swivel bases if he knows fitment of Express vans.
Not cheap though, about $350ish apiece. They're CAD designed and CNC cut, formed, and machined.
Sorry, I should say he's in the process of making them, he's finalized design and sent files off to the machinists.
Here's the thread if you want to check it out:
https://www.astrosafari.com/viewtopic.php?f=69&t=144642