For the Release Preview account, is it worth delaying the request for the account? Our upgrade is not until the end of October. Do they create the RP account at the time of request if we do it late September?

Occasionally Support can re-provision an RP account if you ask. I like to play as soon as possible with RP, but thing could change between now and October!

If you have a sandbox, you can get a separate RP environment for it. Usually, I request a release preview from a sandbox right away but wait until closer to our upgrade date to provision a RP from production to do actual release testing

Thanks for that.