There should be big gaps between each increase in Mlevel, maybe 6+ months or more before we get another. Then similar gaps between each time.
Firstly, the hardcore players will eat everything you put on the table and then ask "ok now what?". Giving these people too much too soon is counter-productive, they have no internal mechanisms to control themselves. So you need to ration this desired content out slower. They won't thank you for giving it all to them on a plate, they will just get no sleep or grind that stuff out like animals and then get mad when they have nothing to do in a month after pulling 20 hour days grinding it all out out.
They are literally like dogs, you give them a weeks worth of food and they will eat it all in one sitting till they look like a beachball. They will then look at you whining for food the next day like you're starving them to death.
Secondly, the slower more casual players need more time to make progress, a lot more. If you don't do this, the hardcore players will all be /99 and the casual players will still be capping out job points.
Thirdly, it adds too much stress to keep pushing forward to get to that cap for everyone. As much as I enjoy gettting these new XP points, I don't like feeling I have to do it and I do feel that sometimes. FOMO is a thing and we are very susceptible to it. I wanna log in and do other stuff too, the faster you keep adding the more pressure we feel to be doing that thing.
It will be a massive mistake to add new Mlevels too fast.
Also I have to say the current system is kind of broken, and I think many would greatly appreciate it if you added an NPC that allowed them to transfer Mlevel (and capacity points) from one job to another. For instance back in the day I grinded merits out on Bard because it was easy to get invites and everyone wanted one. We need this back for Job points and Mlevel.
Maybe add a system to trade CP or EP to Odoro and we get a scroll to use on other jobs? Maybe at a ratio of 3-4>1 (cause yes I get you want to promote people to use other jobs)