In addition to what Bill provided, if you're on 11.6, you're likely on hardware that's not supported on 12.6, so a tech refresh will have to be done. If you planned on doing an in-place upgrade... well, good luck with the upgrade and passing A2Q. With a tech refresh, you can load everything in parallel and test. As Bill already mentioned, you'll need 12 somewhere in between, so you'll need an additional VM with Server 2016 and SQL 2017 to run EDMT so that you can update the 11.6 DB schema to 12. You'll update the schema again in your parallel environment.
I also am going through that at this very moment. Correct you will need an intermediate server with 12.0 on it. What I have found in my testing is that with the tech refresh, EDMT will overwrite the inventory of the destination system with hostnames from the source. I then have to change the machine type I believe to something that can be deleted within inventory, add the components back in, reenable re register SSO, and rerun Web Setup, on pg's and roggers to get them to add to inventory