At what point should side-quests (memories/reflections of strength) be undertaken?

So, as in all SoulsBourne games, there is not definitive "point" that anything should be undertaken. You can play through the entire game without even levelling and dying, so it's more about skill than progression. The only addendum to this is key events and items that develop the story.

E.g. In bloodborne you cannot access Rom before fighting Amelia. You cannot fight Amelia (at the very least) until you kill the Cleric Beast, and Gascoigne. In Dark Souls 1, you need to have defeated O&S before you gain access to the Lord Soul Boss Fights, etc.

However, in Sekiro, you can gain access to a Memory, before you even fight the first Boss (i.e. an encounter with an on-screen Health Bar). Without the basic levels of skill (i.e. the skill tree) or equipment, this would rely heavily on player ability to overcome certain boss fights. Such an example is the Young Lord's Bell Charm, which opens up a memory of "That Night" in the Hirata Estate. The Mini Boss Shinobi Hunter: Eshin of Misen is difficult enough on his own, let alone surrounded by the multiple archers, and torch and axe-wielding soldiers. Having the Mikiri Counter is pretty much a necessity. From here, the boss fights only get harder (seemingly at an exponential rate) until you come up against Lady Butterfly, which in my experience, is twice as hard as the "Main storyline" boss Gyoubu Masataka Oniwa (horse-riding warlord that drops the first "reflection of Strength" Challenge).

So, is there any "guides" as to when/in what order particular boss fights should be undertaken?