Hello! #TeamLogan Here
Picking one Character as a favorite doesn't affect the other relationships, you can get all Logan interactiona without affecting the game. Of course, there are some relationships needed to progress the story and if you only focus Logan's you won't see the content on the other characters, but should be able to prioritize interacting with him without changing much. Bottom Logan has more Lewd content but Normal Logan (top) has romance, and so far the only way to get Hearts with Logan is in his normal route