How to backup iPhone with full iCloud storage?

Hi, I’m genuinely so confused… my phone storage is at 105GB out of 128GB. My icloud storage is at 49.6GB out of 50GB. This is mostly taken up by a backup (37GB). I need to backup my phone one last time before I transfer my SIM card into my new phone, but I am scared if I delete the old backup to make room for a new one, then my data will be deleted/photos gone etc. Is this right? What should I do? I genuinely don’t believe I should need to buy the next iCloud storage upgrade for £2.99 per month as I keep my phone quite tidy and delete things regularly.

[Edited by Moderator]

Original Title: icloud storage backup