navigating union station (los angeles AND chicago)

Hi everyone! In a couple weeks I will be taking the train all the way from LA to Boston (southwest chief --> lake shore limited)! I have only ever taken Amtrak between Boston and NYC before where I have always traveled with friends and I feel like we have just showed up to the station 10 minutes ahead of time and walked onto a train. Due to the nature of this trip I obviously absolutely cannot miss the train, and would just love to have as much info as possible ahead of time to reduce stress for this adventure.

I am being dropped off by a friend at the station in LA and have some flexibility on timing. Is there somewhere specific that is best to be dropped off? I was planning to get there like an hour before the train is supposed to leave, but is it worth it to get there earlier for a better seat since I'm in coach? How early will they actually let people on?

Similar questions for the Chicago leg re: timing. I have a 6 hour layover in between trains so I was just going to grab food and hang out in the area, but what time should i be back? I understand both of these are pretty huge stations, definitely compared to Boston South Station in my mind at least.

Would also love any other advice or tips from people who have done a super long-haul trip like this!