I’d just pay but if you are keen to avoid that then just suck up the nag for the minute. Then over the next few months go through and tidy what you have. I’ll bet there are lots of duplicates, blurry pics or videos you don’t really need to keep.
If you do pay it can be shared amongst your family.
The cost of buying then setting up physical storage will pay the sub for quite a while and is a safer option. You’d need redundant storage in multiple locations to achieve some level of protection against loosing the lot.