Help with customized spawn for every new world loaded [1.12.2]

[erendanet]

So, first of all, I don't know if this is the exact forum I should be posting, and if necessary, I will transfer this question to the correct one. Nonetheless, I am currently creating a new modpack that is RPG based. I wanted players to spawn in a customized prison cell that i've build and I wanted it to load in every new world created. I see two possible ways of doing so:
1 - taking the route that the creator(s) of Blightfall used in their modpack, in that case the world was preloaded and always the same, where you spawn inside the ship. However, this would detour from my idea, which is to have a random world generate (like vanilla minecraft)
2 - (This or something similar would be ideal) The world loads twice, with the first load taking you to the prison cell, (without loading the world chunks) and the second generating the world chunks (caring on the choices you've made for your character at the prison cell, such as class, items, etc). The prison would be nowhere to be found in the world, as it will not be further required to go back into it, as it server only as the inicial lobby.

I lack the knowledge of how to do any of the above. I don't even know if the second one is possible. But if anyone could bring a light on how to do this, I would be very grateful.

Edited March 27, 2020 by erendanet
Editing title to add minecraft version

[Draco18s]