It is indeed sad that he did not get together with his first disciple but his wife is his soulmate. She deserves her position and they are very similar in nature. After finishing the novel it was even clear that they're the one who will end up together. Unlike with the first set of women introduce, the MC literally fall for her wife but due to his mission he choose to ignore his feelings. At this point it is very obvious that she's the one since on the beginning women for him won't disturb his mission but when it comes to his wife, it is dangerous. Plus the way he lowkey care and love his wife unconsciously is very evident.
Dude wtf I just read spoiler from novel that mc doesn't end up with the first disciple. He ends up with a women who apparently forces him to marry her out of nowhere after like 1000 chapters and everyone just accepts.