How much of a nightmare is it to 100% the game, and does anyone have guide recommendations?

Hello all,

Just watched a streamer do a basic blind run of this game and fell in love. I would like to do my own run, and I'd like to 100% it (including all secret bosses and either all-kill or no-kill Oboromaru, whichever has the better rewards).

1. How unpleasant of an endeavor is this?

2. Does anybody have a recommended detailed guide that lists secret items, goes over all-kill/no-kill Oboromaru, etc? I am not worried about spoilers, I've seen the story.

This is Switch remake BTW.

Thank you!