I have (kind of) been where you are, except with another cruise line.
I earned Carnival Platinum status (rough equivalent to Royal Diamond) over a number of years, and the last couple Carnival cruises I took, while still fun, just kind of left me feeling a bit, empty. The quality of the food (while still equivalent or better than the last MDR food I had on Royal, IMO) seemed to have fallen off a bit, the service (especially cabin service) wasn't quite what it used to be, and even at Platinum, the loyalty perks had been cut back, ostensibly "due to Covid". In other words, I'll admit it, they just didn't do quite a good enough job at making me feel "special" - i.e. no chocolate strawberries delivered to the room unless you call and beg for them, the platinum pin is just a generic one now, not ship specific, and you have to go to the photo area and beg for it, the "special Platinum gift" was a cheap $1 keychain, that you also had to go beg for, no chocolates on the pillows for a couple years now, the special gathering scaled way back (lucky to get two free drinks, if you work hard at it), the priority embarkation is now a joke, etc. etc. etc. Yeah, these are all little petty things, I know, but it's death by a thousand cuts to the Carnival loyalty program, the whole point of which, is to make loyal guests stay loyal and want to come back. I'm sure Royal Diamonds have experienced the equivalent of many of these things as well, so I'm not just blaming Carnival - but I just didn't feel that "special" there anymore. Soooo, since I'm not special anymore anyway, why not try something new?
The food on Carnival in the MDR was very decent, but just not as good as it had been in the past, the Vista Class (Vista and Horizon) seemed a bit bland to me even compared to the Conquest and Dream class ships, and friends who went on the very newest Carnival ships i.e Mardi Gras class, came home with stories about how small the cabins were now, and with lackluster reviews about the dining.
So I decided to try Royal, because why not, having heard great things about the Oasis class ships. I tried Allure, and loved it, so now I've booked a 7-day on the Symphony. I was blown away by the size and coolness of these ships. Not having any meaningful status on Royal is a bit annoying after being Carnival Platinum forever (again, Platinum on Carnival is like Diamond on Royal), but the Diamond status will come with time.
I did give Royal an unfair advantage though. The last time I was on Royal years ago, I was just kind of unimpressed with the MDR food, so I booked the UDP dining package. This really elevated the food experience on Royal, by being able to eat in the Specialty restaurants every day, and that, combined with the really cool Oasis class ships, made it a really fun experience again.
I will probably still sail with Carnival from time to time, especially since they still send me offers for almost-free cruises, mostly on the smaller Conquest and Dream class ships (which I think I may like better anyway) through their Player's Club, but I plan to sail more with Royal, and maybe other lines too.
Variety is the spice of life, and I am enjoying the new things.
Point is, I would suggest you branch out, and try new things, and new cruise lines, but don't give up on cruising. Also try the UDP on Royal. It seriously elevates the dining (probably to what the MDR used to be 10 years ago, or even a little better). I know it kind of sucks to have to pay extra for what used to be included, but those are the times, and nice things cost money. Everything is more expensive now, than it was before, and cruising is no exception.
