No problem you say, I'll go and give cron full disk access in this handy GUI full of system preferences with the nice ultra-round window corners. TL;DR, you can't give full disk access to cron. You can try, over and over, but it will not work. You can't give full disk access to lots of other extremely useful binaries that should be allowed to have it if you want them to. You can't even give it to sshd for the love of God. About all you can do is find the buried preference in General > Sharing > Remote Login > (info button) > "Allow Full Disk Access for Remote Users". Which, rather ironically, lets anyone who connects via SSH get full disk access.
Heheheh.
Tonight I fixed the inane system Apple created that prevents me from accessing my own machine with cron jobs: Change that setting, now just have your cron jobs ssh into localhost and everything works perfectly ;)