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https://www.gamedeveloper.com/design/the-reset-problem-a-case-for-single-player-matchmaking
single-player dynamic difficulty balance matchmaking roguelike
In single-player games, players die, restart, and often have to grind through boring/easy content. Players don't get to practice long at their highest skill level. There are some game-design solutions to that: dynamic difficulty, and single-player matchmaking. Specific advice about roguelikes here and here.
https://kotaku.com/how-to-balance-an-rpg-1625516832
single-player RPG JRPG balance classes
Why and how to design your single-player RPG character classes so that they're mechanically interesting. You want players to make interesting choices through trade-offs. Make big differences, play-test, and use spreadsheets to balance.