If you are keeping 30 days and using a 7 Day Full Backup cycle, then you'd need to create 42 days worth of backups before the first backup set can be removed. Only at that point can the first backup set for 7 days be removed, leaving 35 days in storage. If we deleted more, you'd end up with 28 days in storage; under the specified 30 days. To reduce size, assuming your daily incremental backups are small compared to the full backup, you could try reducing the full backup interval to 14 days and leave only 28 days in backup storage. That should cause the product to remove the first 14 day backup set after the third full backup set is complete (at 42 days), while only storing a max of 3 full backups, as opposed to what you have now where you have 6 full backups.

Thank you. In your opinion, do you think doing full backups only monthly is adequate?

it's adequate and safe and normally reduces the overall storage use if that's the primary concern.

We do monthly full backups for all of our file-based backups, since only files that get changed during he month are re-uploaded in full during a "full" backup. These tend to be Quickbooks files, psts, and operational spreadsheets that change frequently during the month. Still, they represent only a small percentage of the overall files that change, so fulls once a month is fine.

Would you say the same for Image Based backups?

I recently switched all image and VHDx Cloud backups to BackBlaze B2 storage. Once per month is adequate for most clients, though some ( who tend to make app changes more frequently) get weekly image/ VHDx backups.
The thing that is not discussed is that you do not need the VM version of MSP 360 to do HYPERV Backups/ recovery. Simply backing up the VHDx files and the .xml files is sufficient to provide for a Disaster Recovery.
For those with very slow upload speeds, I tend to do fulls two-three times per year and incrementals each month, and am waiting for the synthetic backup for BackBlaze to be released in the MBS code.

I have ~100 machines split in ~10 networks doing incremental image to a shared LAN storage nightly, with full image backup weekly, keeping as much as we can hold. There seem to be no way the backup agents can 'taste' how much space is left on the storage location. We also do some cloud file and image backups though less frequent and use B2 as storage. Its important that your B2 buckets are set to keep just a single version - as per CBB support.