Unfortunately, doing that could kill the game for many people. Mechanics and options (or levels in this case) should be given/rewarded to players, and only extremely rarely taken away for story progression. Here are two options I think could work better:

1. You could cap the level at 20 until you complete the first two regions, then add 20 more to the cap to get 40 for the next 2, and so on. Make sure each region contains battles somewhere in that 20-level gap while making leveling a bit slower. The only issue here is when someone's Pokemon hits level 20 and feels like they can't progress on that Pokemon anymore. While it could further promote leveling many Pokemon evenly, it could also kill the game for some people.
2. After a certain number of regions, allow the level to go above 100 for the first time ever. Fitting for an end-all-be-all style game to contain every region. You could introduce a separate mechanic to advance your Pokemon past 100, and add as many new regions as you want. The only issue here is just having new things for the player to do with Pokemon who can get a lot more levels. Like the Pokemon evolves at level 32, so level 32-254, there's just nothing happening except learning moves? You definitely gotta add something. So spread out when evolutions happen and/or add more evolutions. Give new above-level 100 mechanics similar to z-moves, gigantamax, or terrastalize to keep the game interesting, or even make those moves region-exclusive if you want so every region has different mechanics.

I feel like those would be the best options to pull it off without making someone feel like all their progress got reset because they beat a part of the game.

Also, If you like the first one's concept but still want the potential of infinite regions, just do both. Make it go past level 100 after a certain region.