I’d buy a 9207-based card rather than a 9211 at this point, something like this listing or this one. No sense in getting something that uses PCIe2 when you could get something that uses PCIe3 for the same price.

I haven’t bought one of these, but my experience buying stuff from China is that it’s likely to be perfectly fine, and I’d buy one if I needed a SAS card.

I’ve been told that large (e.g. 20 disk) vdevs are bad because resilvers will take a very long time

They won’t really take any longer than on narrower vdevs unless you’re hitting CPU or controller throughput limits.