I have a few questions about the speedrun for new game. firstly I realized that some people use alts so they don't have to worry about glimmer use and legendary shards use and other items like that. Is there a way I can do something like that to ensure that I don't use too much glimmer ect. secondly, why is staying in orbit for too long not allowed

Monitor how much glimmer you have when you start and don't go under.

Because time in orbit is removed from your final time. You could in theory cheese runs by sitting in orbit while waiting for public events

oh what about sitting in orbit for like 5 min so you can take a break

5 mins should not be a problem, I hope. However, post-shadowkeep, There should not really be any reason for you to do anything besides the story missions anyways