I donīt know if Pietro got more like Magdaīs personality because we donīt know much about her in her dialy life under no extenuating circunstances but I agree his appareance along with the knowledge of him being Magdaīs Son does bring a lot of painful memories for him but I think the strongest reason is the brotherhood days what keeps him from trying to get along with Pietro or Wanda, because he knows that even if he was mentally unstable and crazy when he meet them, thatīs not an excuse for his bad treatment towards them or the rest of the brotherhood, I think it says a lot that after he got sane again and got his memories back he went mostly solo in his own dream. I also believe heīs proud of Pietro and Wanda for becoming heros in their own right and while there are times he honestly wanted to get close to his Son despite their past, like in the 80īs or the Genosha era, I think that Magneto believes they are better off in their own lifes than having a relationship with him. Imo this was part of why it wasnīt hard for him to decide to rule Genosha on his own even if this made mad Pietro, Lorna and Wanda and keep being the one thing the world feared enough to get a little off the mutants case than trying again for a closer relationship with them.
Heīs a little closer to Lorna because with her he doesnīt have the brotherhood baggage and they share the same power so there are more reasons for them to interact and still she has her own life, he would like to see her as his heir but he thinks thatīs a decision she has to make for herself, even if sometimes they fight together or agaisnt each other. They also shared the Genosha background and their wish for Krakoa to be different and this would not have happened if Lorna herself was not open to the idea.
In the end what I think Magneto mourns the most is that lost opportunity for him to have a relatonship similar to the one he had with his own father in which they supported each and understood each other very well, I think thatīs what he personally mourns about Pietro but also knows post brotherhood this closeness is impossible not because of Pietroīs fault or his similarities with Magda but by Magneto own actions and in the end he mostly feels like they are better off without him, even if he does love him and his sisters in his own way, he will be there during a crisis but thatīs it and he was content that way, Axis just came to remind him that he really loved them more than even he thought possible and it hurt a lot to get those last links broken, because you canīt teach your heart to believe what your head knows.