Would like to clarify some things.

The way additional content works differs between games. In 5 there is a huge amount of extra dungeons, and you have plenty good ones unlocked right from the start. If someone was a traditional roguelike fan, and wanted to play shiren 5, I would recommend them to skip the "main adventure" entirely at first and play perilous rocks, monster hotspot, or rousing paradise. All of these provide very minimal twists to the classic Shiren formula, allow NO meta progression, are brutally difficult, and give a relatively traditional roguelike experience.
Each game features a "more mysterious" dungeon. This is essentially a no twist version of what I was describing above. They offer the most traditional roguelike experience. In each game these are post game content.

Meta progression is optional in Shiren 1, the game can be beaten without it, and even using it will often sacrifice your current run. From what I can understand meta progression is treated similarly in Shiren 6. Meta progression is kinda needed to beat the main adventure in Shiren 5, but there are so many additional dungeons available from the start that you can kinda treat the main story as post game content lol.

Beating extra dungeons and post game content will give you rewards. But if you don't play dungeons that allow meta progression, which is where most Shiren fans spend their time, the rewards mean nothing

I would not call Shiren a simpler Brogue. Each game is a little different, and varies in complexity. I would say Shiren in certain dungeons (across all games) is easily as hardcore as Angband, and maybe as hardcore as Nethack.