First thing I'd ask you is this, do you use the "optimize iPhone storage" setting in the photos app settings?
If you have not done this, then my advice would be to do that and allow it to take some time to actually do that and you will see a rather large reduction in the storage being used on-device.
If you Have done this, while Kurtosis12 is correct; there are alternative options here. As for whether this suits you is going to be a personal choice. So having said this, if you want to simply have them stored in iCloud and its not important that they all immediately show up in your camera roll then you do have the option to create a folder in iCloud Drive called Photo Storage (for example) and simply upload the older/less frequently used photos into that folder. There is a very distinct difference between iCloud Drive storage and the iCloud Photos "section" if you will.
Hope this helps and if you did not have the option to Optimize iPhone Storage on, when I say allow some time...I mean overnight and on Wi-Fi.
Let me know if you require more assistance