I'm not sure what you were trying to point out. I suggested uninstalling some of the apps that you might not need to gain free space. And I'm not sure why using imovie on your iPhone is relevant.
Anyway, if uninstalling apps you are not using is not an option because you cannot find them (not sure how this is an issue), then yes, trash it and get an iPad with larger storage.
If I use iMovie on my iPhone, I might use it on my iPad. I haven't yet. Probably because it was running out of space. But just offering the iPhone usage as evidence that I'm not anti-iMovie. It's a handy app.
If I don't know the location of a specific app, then I cannot delete that specific app. That is the issue.
The solution option I went with was to erase the iPad and restore from backup as posted in message #17. This helped the space issue. Post-restore, the iPad still seems slowish and I miss not having the iPhone's "3D touch keyboard trackpad feature" (need a shorter name for that, described here: https://www.igeeksblog.com/how-to-turn-iphone-6s-keyboard-into-a-trackpad-with-3d-touch/ )
https://forums.macrumors.com/threads/ipad-4-64-gb-low-on-space.1982947/#post-24271301