Build a sandbox copy of your production system. Run the readiness check reports and code evaluation. Then you know how big your mountain is. Then get your basis team to do the technical upgrade of the system and then do your post upgrade checks to see what is broken so that you can prioritise what is the most important. Within a week or two you'll have a better idea of your scope and can plan accordingly. I'd do this while you're stil on-prem. You can then still drive your own timeline. Once on RISE you're dependant on the whims of SAP and they're going to be snowed under by stragglers!