Reexamine what point in time you need to recover if the data is lost. Losing a week of work is bad in any organization, but may be tolerable to yours. And test restores, which is the entire point of backups.
Your current backups are good in that you have three copies of the data, and one at a different location. Assuming the weekly backups meet your business continuity goals if both the computer and the external were lost.
A storage array improves reliability by surviving a disk failure. Yes, it could make sense to replace the external with a NAS dedicated to backups. Note that if the primary storage is still one disk, it will fail at the same rate as before and some day require a restore. Which may be suitable for your needs.
Beware that an always-connected file share is not a cold offline backup. Ransomware will encrypt such files if it finds them. Assume your backup user is compromised, what damage can it do to backups? Typical file permission schemes cannot easily implement a user that can write to files, but cannot change them or delete them. (Try doing so with SMB or NFS.) Unplugging the backup NAS from the network when not in use may seem tedious and paranoid, but perhaps limiting access to the daily backups is useful. Confirm that the cloud backups cannot be easily altered or deleted, such as using credentials that do not have delete permissions.
Array type is a choice to make, each RAID flavor you mentioned could be decent. RAID 1 of a pair of drives is easy to do, but only 50% usable space. RAID 6 of a handful of drives will survive 2 failures, very slightly slower due to parity calculations. RAID 10 is striped RAID 1, can survive at least 1 failure, and is quite fast, but 50% usable capacity may be too expensive compared to RAID 6. All of the above are better than the single point of failure that is a single physical drive.
Backup to tape could be a different option. Superior in that cold offline backups are easy, stored tapes are not being read or written to. Further, using a different media type means a backup copy has different wear behavior, and is not vulnerable to specific flaws such as in the primary storage's controller firmware. Challenging in that tape library hardware and software can get expensive.