"Can I delete videos from messages without losing them?: I cannot update my phone because I’ve got 7K photos and 500+ videos. My phone says I can free up lots of space by deleting video attachments in messages. If I do that, will I be deleting the video from my photos too?"
-------
Deleting Video form Messages without Losing them:
If deleted, they're gone from the thread. So, check Recently Deleted to recover them. Turn on Messages in iCloud, and then enable Messages on another device. On that device, you can download the videos: Set up iCloud for Messages on All your Devices - Apple Support
Backup your iPhone Today:
Before performing this, back up your iPhone to iCloud or your computer. That way, you can have something to restore it from, should anything go working from here on out.