I'm not sold on the idea of level tiers unless there is a greater system at work. Having your level capped "just because" until you progress the story really sucks even if there is a lore reason thrown in to attempt to justify it. It's also generally a bad idea to build entire systems just to restrict players from level grinding. This is because there are 2 kinds of players that level grind:
- Delayed Gratification Seekers: These players raise their levels higher than an area requires because they enjoy the feeling of easily smashing a boss that is supposed to be hard and are willing to put in the unnecessary investment to have their fun. Restricting these players from being able to out-level enemies directly kills their fun.
- Compulsive Grinders: These players feel compelled to raise their level for a variety of reasons. It could happen because they just like to explore a lot and accumulate too much exp from random encounters. It could happen because the UI shows them how close they are to getting another level and they feel pressured to get "just one more". It could be because they have a poor sense of game balance and think they need to grind extra levels because they didn't figure out the tactical way to beat a boss. These are the players that complain about grinding and call for restrictions.
"Anti-Grinding" features fail far more than they succeed. My go-to examples for this are Final Fantasy VIII and Elder Scrolls IV: Oblivion.
In FFVIII, enemies level to scale roughly with the player as a deterrent to simply grinding your way to victory. The new Guardian Force system and Junction mechanic were intended to replace simple leveling as the main way a player gained combat power. By linking spells to your stats the player could give their party members a large stat boost that would allow the player to out-stat their level-scaled enemies. This system DID succeed in making grinding irrelevant...by making something even more broken.
Now that killing enemies and gaining exp was a direct liability, it encouraged players to simply not fight at all. By simply running away from every fight the player could keep a super low level so that all enemies were extremely weak, then make up for their low level by Junctioning spells to their HP and ATK to stomp everything with no effort. The Draw mechanic to pull spells from enemies only made this worse. Since spells were now consumable (you could have up to 100 of a given spell and your count depleted with each cast) and would lower your stats if cast, the best strategy was to just not fight enemies and draw one of their spells until you hit 100 then run away or use the Card command since it granted no exp. It was very effective in making grinding useless but the devs never considered how horrible the new optimal gameplay loop was.
Elder Scrolls IV: Oblivion also used enemy scaling to deter level grinding and is now infamous for how terribly it was handled. Due to the extreme simplicity of the Elders Scrolls combat system, making an enemy more difficult pretty much amounts to giving it more Health and more damage. And the easiest way to do this was to just keep giving enemies better and better gear. It made enemies more deadly and take longer to defeat, sure, but also left large quantities of extremely valuable loot. This led to combat that was dull due to the simplicity, tedious due to the enemy stats, and needlessly profitable such that players would get more money than they could ever spend and loot itself lost all value as a reward.
By making the high-level gameplay tiresome the devs effectively killed off level grinding and once again replaced it with something even worse. The optimal strategy was to place only skills you DIDN'T intend to use in the "Major" skill slots so the player would never level unless they went out of their way to do so. This left the player free to use their "Minor" skills as much as they wanted since ranking those skills didn't count toward increasing character level. And thus level 1 players could have 100 in all of their main skills and destroy anything the game threw at them and did so because just leveling normally had no upside anymore.
Don't be like these guys and waste time on systems that don't even enhance the gameplay experience. If play-testers are reporting that exp gain is too high and they level too fast just by normal gameplay, then you probably are giving too much exp from normal combat. If over-leveling only happens as a direct resulting of choosing to grind, then you have no reason to stop it.
But from my experience, if your combat system is broken in half just by gaining some extra levels then the problem isn't your exp gain...the problem is your combat system.
A lot of RPGs are vulnerable to this because many rely too much on numbers-based combat. If gaining some stat points makes you go from 0 to God then you have a problem. Even in games where all stats and skills are gained through exp that is acquired from defeating enemies in combat it is possible to make content that is challenging and requires smart decision-making to succeed. Your characters need to have more than just the basic Attack command, and their skills need to be more than just a replacer for their Attack command. Skills should have situational uses, possibly downsides that need to be managed, maybe even cooldowns or interesting requirements to use like needing to be in a certain State or only working on enemies with a certain status effect.
Even a simple Type system like in Pokémon can make a huge difference. Having a big bad level 100 Dragonite means nothing if I have a level 65 Lapras with Ice Beam. Just going by numbers your Dragonite should crush me, but having that weakness to Ice-type attacks means that I can easily overcome the stat barrier by playing smart. And that's just a super simple example.
Some of the harder content in Fate/Grand Order can be downright mean and requires strategy to stand any chance of victory. In the beginning it's easy to power your way through by getting a 4 or 5 star unit and just leveling them up a lot. But all that gives you is raw stats and even leveling your skills only increases their numbers slightly. The challenge bosses require thoughtful team compositions, with what kinds of abilities a unit brings to the table often out-weighing their raw numbers. Different challenges are designed to screw over different team compositions, so one challenge may call for a glass cannon team to DPS the boss down in 3 turns while another challenge may require strong healers to form a team that can stall long enough to chip the boss down. By paying attention to what players are using a relying on the studio can make challenges tailored specifically to the current meta to get players to break out units they may not have considered before. Pay attention to your play-testers; if they are relying on a specific skill or combo to crush so hard, consider making enemies and quests designed to counter those strategies to force them to look at other options.
If you are hard set on a level cap, then I recommend you look into Star Wars: Knights of the Old Republic. The game is built on a D20 system and actually has a level cap of 20 for the whole game. This turns out pretty effective since it makes each level up very special and lets each level provide significant power (you can get stat points, skill points, and points for new Force powers all at once). One of the reasons this works is because the total amount of exp in the game is limited so it encourages the player to explore around and get every last point. But it also requires the game to be very rigid in how much you are allowed to level at any given point so it does remove some player agency. With a good enough skill system this could still work for you though.