How to auto delete iPhone backups in iCloud to make space for new backups?

I have a specific question, then a couple of general issues at the bottom.

Right now I'm on the 200gb plan. I have not backed up my iphone in a while and I'm trying to do that now. The whole 200 is taken up by one previous iphone backup. So I had to manually delete that backup to make space for a new backup.

Is there a way to have a new backup just automatically replace the previous backup? I certainly don't need more than one back up, if I lost my phone I would just restore from the most recent back up. So for me the new backup should always just replace the last one in my opinion.

Is having to delete it manually the only way? Basically I can't backup unless I go do that manually every time I guess.

Furthermore, I have deleted the previous backup to free up space, but the new back up is still failing. Does it take time for the iCloud cache to propagate or something?

Honestly I find iCloud very confusing to manage.

There is a smaller amount of storage taken up by "Mail" and "Documents" which I would also like to review and delete. But I don't see how to do that. (Looking in desktop OS iCloud account). I see the Mail app, but nothing to really manage. And where are "Documents"? Is it all managed from IOS? See attached screenshot please.

Thanks for any tips.