Also........... Yuuki look at your life and choices. Porthos joins you in the 'wtf is up with these journal things' tbh, even if he does also have a healthy dose of curiosity about it.
It really depends, for Porthos. There are a lot of things that he really does like about Kiseki... and things he doesn't like. Ultimately, he knows it's not a world that could permanently "home". He misses Athos and d'Artagnan. He misses Paris, for all its faults. But here, he knows and loves Robin and they can be together. Here, he knows fully just how much he loves Aramis. Here, Aramis can be happy with his son and with Anne and have his family and be able to interact with them and spend time with him without danger. Aramis can be with him without there being danger of being found out and killed or discharged from the regiment. He can learn things and go to school as he pleases. While far from perfect, he hasn't experienced the prejudices and "ism"s of his own world.
But for all its faults and shortcomings... Paris is still his home. He'll always want to return to it, no matter how wonderful another place might be, whether Kiseki or the other worlds he's been learning about from others.
But he'd never want to leave it if it meant everyone else - especially Aramis and Robin - were left behind without him. If it meant that he could never return again, or they had to stay. Or, vice versa, he'd never want to stay in Kiseki if it meant that Robin and Aramis would leave and never return. Basically... his home is where, ultimately, his brothers - and Aramis specifically - are. He'll always want to be with them, no matter the world or outcome.
Say Yuuki got a Mokona that allowed her to travel between worlds. Which world (aka, which person she developed CR with) would she want to visit first?