I haven’t used open wrt before and need to buy a comparable router. I’m also not sure if I might swap to ddWRT (and I’m not really sure what the difference is)

The Netgear R9000 is supported by ddWRT but not on the official list of openWRT routers. However, there is an active git for the firmware and the git seems to be a mirror of the official openWRT

So does this mean that openWRT is available, but in dev for the R9000? And that I could use it but might have issues?

The reason I chose the R9000 this is that I was looking at the R500 and can get that pretty cheap and works on both openWRT and ddWRT. But I found a R9000 for the same price as the R500 and the R9000 has double the ram and flash.