As I understand it, you will remove the bridges, and one-way portals will lead to the central island. On the island, there will be an entrance to a dungeon, which will contain four more one-way portals leading to the individual dungeons of the four players. In this case, the player will inevitably go through all the "goodies" that are both above and below (the problem with the current version is that there is no point in going to these artifacts because defeating their guards is more difficult than defeating the player himself). I still believe it is necessary to weaken the mobs - the question is not whether this map is "for hardcore players or not," but whether there is a need to engage in combat with the mobs - there isn't right now for the reason mentioned above. Reduce their strength by 1.5 to 2 times.
If I understand your idea correctly, then yes, I like the idea with the portals, but I don't see the need for keymasters - it will be enough to simply place guards (not too strong, as it could happen that the player gets to the central territories and cannot get out of them).
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...