An on-premise backup is the most cost-effective solution for long term backup retention. We would advise using a NAS to store local backups due to their ability to store a large amount of data for a low cost.
Multiple retention policies can be applied to this local backup data so that you can have both short term and long-term backups.
A long-term retention policy, often known as a G-F-S or grandfather, father, son backup, or backup archiving can be useful if you need to retain backups for a longer period for compliance or regulatory reasons.
On-premise backups do not protect again on-site events, such as fire, flood or theft, so we would recommend pairing your on-premise solution with an additional cloud solution. This is called a hybrid backup solution