Which bosses can only be fought in certain endings?

Sekiro, Shadows Die Twice, has four endings all together, and not all of the endgame bosses are fought in all the endings. So in order to fight every boss, you either need replay the game multiple times or make backups of your saves. I've decided to go with the former, and as such I'd like to know which bosses are fought in which endings so I can take the most efficient path possible to fighting all the bosses.

Which bosses can only be fought in certain endings, and in which endings do you fight them?

Ignoring the bosses that can be encountered regardless of ending (everyone you fight before choosing sides), here are the bosses that can only be accessed via specific endings:

Shura Ending:

Emma, the Gentle Blade

Isshin Ashina

Immortal Severance / Return Endings:

True Monk

Divine Dragon

Demon of Hatred

Genichiro, Way of Tomoe

Isshin, the Sword Saint

Purification Ending:

True Monk

Divine Dragon

Demon of Hatred

Genichiro, Way of Tomoe

Isshin, the Sword Saint

Owl (Father)

One thing to note, while the fight against Owl (Father) is part of the Purification ending requirements, it is still possible to challenge him and choose the Immortal Severance or (if the required items have been acquired) Return endings after defeating the Sword Saint.

Additionally, the fights against both Emma/Isshin and Genichiro/Sword Saint are back to back and only reward one boss memory.

The only ending that has any extra bosses is the Shura Ending where you need to side with Owl and then you fight Emma and Lord Isshin.

In the other 3 endings you need to side with Kuro and defeat Owl and then mostly collect some items or trigger some events. In the Purification Ending, you need to defeat Owl a second time in the Hirata Estate which I guess counts as an extra boss.

The different endings are mostly about doing some extra leg work and not much to do with extra bosses.