One popular bl isekai novel is 'KinnPorsche: The Novel'. It takes the story of the two main characters and transports them into different situations with elements of the isekai genre. Another is 'Love by Chance: The Novel', which also has bl themes combined with unique isekai - like plot twists that make it interesting for fans of the genre.
Well, 'TharnType: The Novel' can be considered as a bl isekai novel to some extent. It has a story that involves the two male leads in a world that has some elements of being otherworldly or different from the normal world, much like the isekai concept. Also, 'SOTUS: The Novel' has certain aspects where the relationship between the two male characters develops in a setting that can be related to the isekai genre in a way.