[Borgbackup] Option to keep yearly backup counting from first backup?

[Clemence Magnien]

Hi,

I hope this is the right place to post this question.

I've been using borg for a while for backing up my home directory
and am very satisfied with it.

I'd like to keep:
 - 4 daily backups
 - 4 weekly backups
 - 12 monthly backups
 - 1 backup per year for all years since I've started using it.

As I started using borg in may 2017, the keep-yearly option of prune wants to keep
the latest backup in 2017 and remove the ones before that.

I however would like to preserve as much of my history as possible, so
I would like to keep:
 - one backup for may 2017
 - one backup for may 2018
 - 12 monthly backups for 2019
 - (and 4 weekly and 4 daily recent bakcups)
 
Is there a possibility to count the backups you want to keep from the time
I started using borg rather than counting one at the end of each calendar
year?

I know that I can use the --keep-within option but it seems a bit convoluted.

Best,
Clémence