database – How to store large amounts of data in a custom field

This question has already been asked in the same way, without any real conclusion.

LONGTEXT max for MySQL has 4,294,967,295 characters.

But wordpress refuses to save a custom meta of 2,000,000 characters.

Specifically, I encrypt AES binary image data and try to save that string as a custom field via a front-end publication form. Or convert to base64 and encrypt. I can submit the form and create the message, but the custom field is empty.

I have searched everywhere for an answer and, with my limited knowledge, I still have to think that I can either

A: limit the size of images before submitting
B: Divide the encrypted string into pieces recording each song in separate custom fields

My first question is: why can not custom fields save 2,000,000 characters?

Is there a specific way to format a channel to exclude it?

Is there a better way to do all this with the encryption of image data required?