I use an old iPhone supplied by my employer so it does not have tons of storage space.
When I receive messages that my storage space is running low I go to my iTunes app and try to rent a really big movie (I generally choose the Lord of the Rings trilogy because that is enormous). My phone then tries to clear out space to download it, before giving up and letting me know that there is insufficient storage. I generally do this two or three times, which frees up to 5G of storage without deleting a single thing on my phone. And without me having to actually download anything.
I find storage space less of an issue with the newer versions of IOS because you can ‘Optimise’ the storage on your phone automatically and leave things stored on iCloud and only download them when you need them. You can also ‘Offload’ apps when you haven’t used them for a while, the icon is still there, but not the app itself, it downloads it if you need it.
I have an ancient iPhone 5S 🙁