I meeeaaan, Isshin was very close to Sekijo/Monkey/the Sculptor, just swap him for sekiro in your theory and it's pretty much canon already. We know Isshin adopted Genichiro and afaik we don't see any het partner implied for Isshin sooooooo...

It also already works with Isshin having to sacrifice & save Sekijo from Shura and abandon that relationship for his dreams of ruling the land - he chose his ambition and now in his old age reminisces about such things as, in the moment, hesitation is defeat. We already see the events of Sekijo's life repeated in Sekiro's, and he is very much a thematic mirror of the older shinobi.