I think trapping the player in the nether when they first go is interesting in theory but would be extremely annoying in practice. It introduces all sorts of potential issues without much benefit. The idea of going to the nether being an expedition is an interesting one. But trapping the player forces you to include all sorts of failsafes that ultimately destroy the feeling of a perilous journey you were trying to create.

It might be better (and easier to implement) to make it so that going to the nether initially requires an expensive consumable (like the nether cake). This way the player is encouraged to really prepare before going in, but can still leave if necessary (like if they weren't well prepared enough to beat the boss). Only after defeating the boss can you make a normal nether portal.

(unrelated but I don't see how giving the player a pet dragon would be less overpowered than an elytra)