Okay, so we've got a good confirmation that beastgirl is the genki harem member, the elf looks like the ara ara onee-san harem member, and the last one is the younger little sister harem member based on nothing more than just their appearances.
Also, why bring it to "Oh I studied this beginner quest stuff a lot" and not "Oh, I have literally 7 lifetime of memories of my youths where I did this basic quest and it all culminates together into this array of small details and knowledge that is all encompassing."
Though we have a very clear view of what the author has chosen for the MC's amalgam state of lifetimes. That is to say that he is ultimately just himself with superficial knowledge and skills.
The concept COULD be cool… imagine a version, where the skill holder is basically “cursed”. They gain the memories, skills, knowledge and all from a past skill user. But that power acts like a magnet, and forces demons to
always be drawn to them.
You could have a slow reveal, like “oh everyone who had this power has been a hero!” which turns - eventually anyway - into the revelation that each hero was forced into the job. They were heroes because they were under constant threat. They traveled the lands, hunting demons, because staying in a city would bring its end.
And the final aspect of the power, is that once you die, you will become part of that knowledge.
I’m thinking of a story like this, but the power is a little bit of One for All from MHA, and a little bit of Ozpin from RWBY. The “hero” can talk to, and gain wisdom from, the past users. And gains their power. But will slowly be taken over by the power, and will need to live away from others forever.
The story could be about the character deciding that “being the perfect adventurer” is worth it to him. Or maybe it’s not, and he fights against that destiny. Or something else entirely. Could be cool.