There's a lot of good stuff in this thread that I'll try not to repeat, although I will note that synchronization is more of a mirror of current state than true backup. The problem with synchronizing is that if you're not careful, you can be vulnerable to accidental overwrites and deletes, especially overwrites. If you change a file, and then run a backup, then if you find that you need the older copy of the file, that one may not be recoverable. A simple way around that is to set your synchronization to not delete older stuff, but as you've noted, the problem with that is that you eventually fill up all the available backup space with a lot of clutter.
The easiest way around that is in periodically making snapshots of old backups, so that you have copies of older stuff. How often you do that and how many generations you keep is really the same question as how frequently you back up -- how much data can you afford to lose?
If you want synchronization, I think you can do a lot better than SyncToy -- the design is old, and I don't think it's really intended for large-scale synchronization that you're doing with your backups. For several years, I used that to synchronize a few GB of data from my hard drive to a traveling drive. It worked for me, but I found that it wasn't especially robust. I eventually transitioned to FreeFileSync, and once I got used to the differences in UI, I've found that to be a lot sturdier. The UI indicates that it can be used for larger-scale backups, although I'm not sure I would do that.
Robocopy has been mentioned as an option, but I think it may have some of the same issues I've noted. To me, the main benefit is that Robocopy will do copying that preserves ownerships and permissions on files. That can be useful if you're doing large-scale copying of system files (although that doesn't get you bootable media).
Moving to a tool that is expressly designed for backups may be the better bet for you. Some use synchronization, but not all. A lot of tools are database-based that allow tracking of multiple copies over time, but not indefinite retention, either. If you're using something that is tracked by a database, then as with synchronization, your backups are updated where only stuff that has changed since the most recent backup is written (media size and bandwidth). However, you have the ability of being able not only to do a full restore from the most recent backup, but where you do a recovery based on a specific time index, including the ability to get to earlier versions of files that have more recent backups. You may not need that kind of thing frequently, but it's nice to have on occasion.
Personally, I'm making use of Duplicati, for a variety of reasons. It's not a sync backup, but it does have snapshotting capacities, and corresponding ability to do recovery by time index. Duplicati is multi-platform, and I use it on Windows, Linux and Mac OS, and it has the ability for me to write to a networked drive, whether LAN or cloud, if I choose, and encryption of backups. Because it's open source it's free. It's worth noting that most commercial backup tools will not allow you to back up to a network drive on an unpaid tier.
One commercial tool that I like that's free for individual users is Veeam, and I provide support for this for a number of users. I believe that it's sync-based, but supports time-stamped recoveries, and where it's possible to write to a network drive, supports encryption of backups and also to do full image recoveries. Veeam prefers cloud-based backup where they can sell storage space, but for individual users, it's not a problem to write to an external hard drive with the free version.
Speaking of encryption -- if you have a tool that encrypts your backups, you can write to pretty much any device that the backup tool can address. Conversely, if you have encrypted media, then you can use nearly any backup tool that you want, and still have encrypted backups
Several other backup that I know about, but haven't reviewed: AOMEIBackupper (supports full-image), Cobian Reflector, Iperius Backup, Uranium Backup. I think that many of these are not sync (.ZIP archives), although I think most support encryption. However, if the tool itself adequately supports the version management process, you may find that you don't need a sync approach as much as you think.
I'm not being critical of sync -- there are plenty of use cases where it's definitely the best way of going. However, if you use it, it's wise to know where the trade-offs are, and where a sync approach can have problems.
Ultimately, an effective backup regimen relies not only on multiple media, but multiple methodologies, and even multiple tools. Depending on what your recovery scenarios might be, you probably need more than one approach. It's entirely different to be recovering a few files or folders (from an "oops" overwrite or delete) as opposed to a full system restore as a result of a disk failure.
