I have the same requirement and I solved it as follows: Apply a label to emails dated after the last backup, using an “after:YYYY-MM-DD” search. Takeout then o

I have the same requirement and I solved it as follows: Apply a label to emails dated after the last backup, using an “after:YYYY-MM-DD” search. Takeout then offers the option to export only that label. I do an annual backup so the amount of manual effort here is acceptable.