I like the idea of MB being a totaly end game content for few best players, but I realy think that requirements should be different for Diamon and Platinum. Don't get me wrong but double experience and +1 to catch multiplier gives huge advantage after you put it all together.
In my opinion experience requirement should be at least 50% higher for Platinum (with exp ticket Diamond has x2 rate and Platinum x3 so 50% more sounds fair to me). Not only you get experience faster but Platinum is also less populated what makes it much easier for people to find a decent place to hunt in peace.
I have no idea how to fix catch matter, it doesn't make much of a difference for very rare catches since both worlds most likely have to get to the bonus rate but starting from rare the easier catch it is the faster you will get it on Platinum what makes grinding to 650 a lot less time consuming. At this point I don't even feel motivated to try catching when I see people throwing 2-3-4k balls on some random "rare" pokemons that doesn't even have bonus rate like Skiplum, Flaffy or Nincada.
About limiting catch possibility, I don't realy thing that reward for such high effort achievement should have any limitation other than 1 use per character/account. Saying that "mythical" pokemons are going to be excluded is like telling me that Catch Charm > Master Ball. 0 effort to get it (just cash) and time to spend on the spawn. Sometimes a lot of time like in Billy or Khormes cases but surely less time than getting all requirements for MB.
Overal the whole idea sounds realy interesting and it is something that could make me start playing seriously again but not in current state.
I respect that you acknowledge some of the very rare pokes are hard to catch where it doesn't matter on which server it is. This will probably start a shitstorm even if I didn't intend it, but not in a single aspect can I agree with your point of view.
Let's start with the question, why does it matter if diamond and platinum have different requirements considering the MB quest? Would it be abusable? Can some people really grind these things on their alts concering the server rates? I don't think so. Can it be traded via servers? Does it also matter because diamond to plat rates are 1:2 so it would still be double the cheaper compared to diamond's, without looking at the current inflation that's happening on platinum.
Now let's get to the point of requirements. In which perspective do you really think platinum players have it easier? Where there are more free spawns so people can focus on exping? What kind of argument is that, pretty nonsensical when you know yourself that people who play PA have different goals, some may focus on leveling and reach the level 450, but what about the catches? If he's not a catcher, do you think he would manage to get 650+ catches in a short amount of time? As a catcher myself, my goals in PA are in this order: catches>cards>achievements>collection of pokes>others. So when my main goal is to "catch'em all" do you really think I would fry my brain with countless hours of exping when I haven't spent a day in PA for the past 10 years focusing solely on experience of the character. Even with me constantly focusing on catch, my current 465 is not that high considering how much I have no-lifed or camped some of the current spawns, since of course that's my main goal and that number is the most important one for me in this game.
Turning back to the other requirements. Okay let's say some people do focus on both catch and leveling, what about the rest of requirements? Cards? Since I've been gathering cards as well, and I know how painful it is to spend hours to try to drop a regular card like abra or drowzee (yes, I've spent days trying to drop each of them), I don't think that requirement is easy as well if you consider people don't do achievements like I do where you can get "free" cards.
About tapus access I don't even have to talk about, I wanted to stab my eyes with forks while doing decidueye task, yes primarina is easier now but I camped 3x spawn of it also. And I haven't even seen Lele's tasks yet.
Achievements +-, it depends how many and how hard they are for the MB quest.
And then there's this zygarde cells, since I've heard about it, I haven't cared in the slightest about the cells, and I can bet that 95% of the servers would agree with me on this one. Even when all cells are released, do you really think that these 95% of people would suddenly say "okay time to hunt zygarde cells", even if that is the case 50 zygarde cells is not a joke. Some people play this game casually, not sure if I can consider myself as one since I tend to find the game boring if I'm not trying my best, but I also play for few months then rest from it because a lot of things simply bore me and I don't feel like doing what I originally planned. And when you look at total of all this sum I can literally name less than 5 players on platinum that would actually try to get the MB, and even if they did manage it would be totally deserved (you can make jokes that we don't even have more players than that).
Now let's do the retrospection, why do we need to adjust the requirements specifically for platinum? The way this is mentioned I can only see it as envy towards platinum players where they have it easier so let's fuck them up by making it harder. If I decided to play on platinum to take it easier, why didn't you too? If you feel like struggling go ahead keep playing there, but don't point fingers at platinum players just to ruin something for them, if this post was to make things for diamond easier I'd understand, but you're just trying to make it harder for platinum for what reason? It doesn't even concern you since you don't play here. And you don't really have the right to comment on what's easier and what's not since you haven't played on platinum, most of the game is RNG anyway, rates don't make much of a difference.