The sixth Shinwa Densetsu no Eiyuu no Isekaitan novel curiously does not deliver on the dark foreshadowing of the fifth and instead appears to mark a turning point in the story where our protagonist (presumably temporarily) leaves the stage. It’s fine I guess, if a bit rushed.
I had first skipped over Ichioku-nen Button o Renda shita Ore wa, Kizuitara Saikyou ni Natteita (English subtitle: ~THE UNBEATABLE REJECT SWORDSMAN~) due to the idiotic setup, and afterward by conflating it with Mugen Gacha. Eventually picking it up with the reasoning that the billion years of training could be ignored so long as it wasn’t constantly referenced I discovered that, essentially, it’s just a massively inferior Rakudai Kishi no Cavalry. Go read/watch that instead.