Omega Dynamic Geneve for 8 inch wrist..

Agreed. I don't think they made them that big! Fortunately, they are actually quite easy to make, and plenty of Etsy sellers make them.

I'd suggest finding someone who seems to make/sell them in a material that you like, then just have them make one for you in a bigger size!

I did a quick look... this one might be close? :
https://ziczacleather.com/product/b...yment-strap-for-omega-dynamic-geneve-wt12582/

215mm is 8.5" long. Buckle is going to add 3/8, and all of that probably gets you at least to the last pin hole.

None other advertise the length, but you might want to ask any of the sellers!

As far as 'needing a ring', the strap itself has a circular cutout (see the picture). There is a screw-on-ring(looks like a screw on caseback, except it doesn't have the center piece obviously) that holds the strap or bracelet to the case that you'd remove, put the strap in place, and tighten it back down.

See this photo here. The ring unscrews, and the strap/bracelet is sandwiched between it and the case (pictured lower center). You can see the center ring on the bracelet.