Recently, I was thinking that overleveling spoils the game and that some level cap feature will improve the game. Then it occurred to me that I can implement this feature by myself. In online mode, the save files are stored on the devs servers (so, you cannot cheat). However, in offline mode, you can edit your save files as much as you like. For example, you can edit your levels to fix the overleveling.
I found a steam guide for editing save files. It turned out that the save file is an SQL table, which you can edit with an sql-browser. After editing, the save files are ok, and the game doesn't crash. So, you can implement level cap by manually editing your save file after each mission. But this will be inconvenient when there will be more team members, beasts, and robots. So, I spend a little time to level up my coding skill and wrote a python script that does the job. You still need to launch it after every mission, but it is less than 30 seconds (mainly loading times). I think this is a reasonable price for such a feature.
To test this feature, I restarted my fourth run. Currently, I'm at the beginning of the chapter 3, and I'm having a lot of fun (despite the fact that this is my fourth run). The game is really better with level cap. The most appropriate comparison is the low level Dnd/Pathfinder game: at this stage, the game is simple in structure but has respectable difficulty. I needed to restart even some ordinary cases. I cannot send Irene in hero-mode for a long period of time, I need to return her for healing. In some missions, I cannot allow my self hero-mode, and need to keep the party together, which forced me to use a more party oriented build for Irene. In previous runs, I received little damage, so I used Anne as a witch. But with level cap, I need healing. In previous runs, the only intrigue about Ryo was whether he will drop his sword or not. But when you approach this mission at lvl 14 and Ryo is 30+, he is a dangerous target. At first attempt, he just raped my party. On the second attempt, only Sion survived. Overall, I confirmed my thoughts. The game has very well designed missions, but some of their beauty is lost due to overleveling.
Recently the devs announced that they are planning to introduce level cap option in the future. So, probably, in a month or two they will add it. I think, when they officially introduce this feature, the game will become even better than it currently is.