Let me rephrase.
It's not the thickness of the seat pad, the problem is a
cutout in the seat pad of the "Gesture" which can lead to
pain in the tailbone region, like he explained in the linked video.
In their
Best Office Chair Tier List (30 Ranked For Comfort) video review the
"Steelcase Gesture" ends up only in the A tier row, so it's overall not a bad chair.
But the seating comfort is just the most important aspect of a chair, it's the part you have the most time contact with. The
"Steelcase Amia" has the best seat pad out of all 30 chairs tested, which is why it would be my #1 pick. The "
Herman Miller Embody" looks also like a solid choice, but it's not available in leather, so it's out of the window for me.
About customization, I don't know where you're shopping. But over here 2nd hand is "buy as seen". There is no customization possible like when you buy new. And just to be clear, with customization I mean frame color, fabric type/color options & wheel types. To make it fit into the rest of your furniture, you know.