Good reason to convert LONGTEXT column to smaller width if field is smaller?
Let's do the math.


6500 characters, even in CHARACTER SET utf8mb4 takes no more than 26000 bytes.
TEXT has a limit of 64K bytes and needs a hidden 2-byte length field.
LONGTEXT has a 4-byte length field.
Let's say (for the 'math') that the average row length, including this text, is 3000 bytes.


Math...


Savings of switching from LONGTEXT to TEXT: 2 bytes per row
Percentage space savings:  2/3000 = 0.06%
Performance improvement:  Less than that.


Conclusion...  We have spent much more effort than this minuscule improvement is worth.  (But you probably learned some details.)