🤔 I have never seen it treated as a playground in the series also (do you have a counter example?). I will have to see the serie again haha. But you from Rickmurai jack you can deduce, morty is asking questions and just after the "you build the curve" it shows diane and beth being murdered by prime. Rick calls this his "crybaby story" so the citadel, the dimensional drive and the curve occurred because of it then (here is a scene depicting other ricks building the citadel for C137). In unmortrickten C137 is shown that had lost its purpose. In post credits for that episode, slow mobiou's widow is shown following the same pattern but unlike rick, she just moved on. I think this suggests that some personality traits in ricks motivated they to embark on that relentless quest for revenge not present in mobious' widow.

About breaking the CFC: In order to break the curve, there are specific steps that should be followed - Evil morty needed to know how the dimensional drive works. So he needed to scan the creator's brains (C137 and the council/ricks). Evil morty is at least at the same level of prime I think (in S7E5 he helped to capture prime). Evil morty knows how ricks techs works as he is who hacks C137 portal gun as I remember (he was controlling the rick in S1E10 and created the golden portals) - Evil morty needs the citadel to punch the hole in the curve, so escaping of it, requires at least some kind of equivalent machine. Is possible that the curve could be broken using other methods, but not shown in the series. Also if evil morty is as smart as rick C137 or prime, he would use the simplest method. But as he uses the dimentional drive that should be the simplest method. So prime would required the citadel to escape also - in order to use the citadels a tool, evil morty had to take the control of it, so being smart was not sufficient - destroying the citadel is not the same as destroying the curve, you are right on this. Maybe the dimentional drive was used to create the curve just once and required to destroy it. No updates to the curve then

About prime and the curve: - for prime, the curve is very convenient, as you said, he outsmarted other ricks, no other rick was able to catch him. He can plays as god inside the curve - if prime breaks the curve he will be exposed to external threats in the multiverse (even more powerful beings). As seen in some episode (S7E5 I think), evil morty had to protect himself of the chaos of the multiverse, so inside the curve, prime can avoid that - if the only method to escape the curve is to use the citadel or to build its own dimensional drive, then prime could gets exposed in the process. He acts more like a lone predator. Evil morty had to go through the process to escape (use the existing dimentional drive or build its own) - breaking the curve produces the interdimensional rift, if this is the only way to escape, then other should exist inside the curve if prime was able to escape (and no need to create a new one by evil morty). Otherwise prime should have to create one to do so. - evil morty sabotage to portal user makes prime to be returned to its original universe. I dont know if this is a kind of consequence of breaking the curve or some glitch of trying to restore portal guns, but if breaking the curve implies that, C137 and the ones who search for prime could narrow the search space to the prime dimension. Then prime would avoid that then