Convert Differential Backup to Full?

Hello,

I'd be grateful for some help with differential backups.

The way I backup my music library is an initial full backup then
differential backups every year that are rather small because I only
add a small amount of files to my library over time.

However this year I found that the differential backup turned out to be
95% of the size of the full backup because I edited the metadata on 95%
my music files so they were correctly regarded as changed by dar. So
the backups I have now are:

1st backup (full) = 300GB
2nd backup (differential) = 5GB
3rd backup (differential) = 5GB
4th backup (differential) = 5GB
5th backup (differential) = 290GB

I want to avoid essentially storing my library twice. Is there a way to:

1. Delete backups 1 to 4, AND
2. Turn the 5th backup into a "full" one, and backup the remaining 25GB
of my music library that hasn't changed since the first backup.

Essentially, to avoid doing a fresh full 315GB backup, when I already
have 95% of the final product with the 290GB backup.

PS: Thank you for this amazing tool! Been doing backups with dar since
2018!

King regards,

Daniel Bedrenko.