Whenever you can think of some specific content-related structures immediately (e.g. financial vs. non-financial) then use those ones to group your thoughts.
However, if there is nothing comming to your mind like that, also grouping your thoughts into e.g. 3 main buckets is perfectly ok.
Bonus 1: Any structure is better than no structure at all. Maybe your generic 3 buckets are not as perfect as it could have been, but it's much much more important to having any structure than no structure at all.
Bonus 2: Once you have your structure, keep in mind that there is also a communications dimensions. Having a structure and communicating in a structured, top-down way is not the same, but the communications part ensures what and how it comes across.
Bonus 3: If you can think of a "rigid structure" like you mention it and you still have some more ideas to add which don't really fit into any of those "rigid" structures, feel free to add a bucket "Others" or "Additional aspects" etc.
A related question was asked on Quora a while ago here - I added some more details in conjunction with structuring case interviews: https://www.quora.com/How-should-I-use-frameworks-in-a-case-interview/answer/Robert-Steiner
Hope that helps - if you, please give it a thumbs-up with the upvote button!