Once you achieve Diamond on RCI you will be tagged as Elite when you sail X regardless of how many points you slowly accumulate on X.
If you make Diamond on RCI and then never sail RCI again (devil speak right there) and your goal is to achieve Elite Plus (750 points) you will only get there when you X points naturally reach 750. Since you are starting at zero that could take awhile but at least you'll be treated as Elite for all cruises during that journey.
I'd argue Sky Suites are more Grand Suite like. Full day access to suite restaurant and Michaels Club suite lounge with free drinks at night. Also consider only Oasis and Quantum class have Coastal Kitchen so when on older RCI ships there isn't a suite restaurant for any suite level while more of the X fleet have a suite restaurant.
Cabins vary by ship but even on the older Millennium class there are some very nice Sky Suites. I wouldn't compare these to a JS on an RCI ship of a similar vintage.
This strategy wasn't lost on me. I enjoyed my X cruise but RCI ships are younger at heart. I can accumulate status on RCI knowing that X is there waiting for me as I continue to grow up and mature. When I'm ready I can sail X and at worst be Elite even though I have just 112 X points as my base to start from.
