How to keep backups if I am away from the PC for a very long time

#### My backup strategy:

* The btrbk systemd timer **automatically** creates one backup per day on my different disk.
* I want to keep **30** backups (snapshots) on this disk. (30 backups / 30 days)
* If backups are more than at least 30 number, the oldest backups should be deleted **automatically** until exactly 30 backups are kept.

My config:

```
timestamp_format long

snapshot_preserve_min latest
snapshot_preserve no

volume /btrfs
snapshot_dir /btrfs/@snapshots/btrbk_snapshots
target_preserve_min 30d
target_preserve no
subvolume @home
target send-receive /backup/btrbk_backup/home
...
```

#### The issue:

**If** my PC is offline for 2 months long, but all snapshots are older than 30 days and are deleted automatically when my PC is on.
They would be gone. No backup.

Have you any idea how to keep 30 backups (snapshots) regardless of age?