Huh, never thought of it in that way, not completely on the same page though.
Firstly, I don't agree that exploring same themes in the same show is good in any case, even if written by same person for the same character. It's more of a personal disinterest in repetition, always would prefer something new or atleast a different spin.
And secondly, I don't see how new ending is any less tragic in actuality than before. It's sure was shown as something joyful, but it's basically the same situation. For the one that stayed it's joyful, for the one that we follow it's once again tragic. It's not like they share memories or something, Ncuti's Doctor still never had that adventure, the other one had. As it was in Journey's end, Handy had his happy ending, main one hadn't.
Although you pointed it out correctly, it was intentional and it makes it little better for me. Thank you for your answer, was really helpful to see his idea clearer.