How hard is it to visit the Colosseum?

First of all - sorry if this isn't in the right section. Feel free to tell me where if I am in the wrong place!

I'm planning a trip to Rome from 3 to 10 September and really want to visit the Colosseum. We've been looking into getting tickets for the Full Experience, but it seems like they're all sold out on the official platform. I was hoping to get some advice from anyone who's been in a similar situation or has experience with visiting the Colosseum.

A few specific questions:

  1. Is there any way to still get tickets for the Full Experience? I understand that it's popular, but are there any tips or alternative ways (trusted resellers, guided tours, etc.) to secure these tickets?
  2. I read that you have to choose between visiting the Underground OR the Attic with the Full Experience ticket. Is that correct? Is there any option to do both, or do I have to pick one over the other?
  3. What is the different with "24h – COLOSSEUM, ROMAN FORUM, PALATINE" for under 25 years old which costs just two euros? what kind of visit do you get there?
  4. Any general tips for visiting the Colosseum? I didn’t expect it to be this difficult to plan, so any help would be greatly appreciated!