How to clean system files? (MacBook Air 2018, OS -- Mojave)

Hello dear everyone

I know that managing system data is an old topic, but I am not a big Mac user either. This Macbook is the only Apple device I have (so, no other devices are available). I use a relatively old Mac Book Air with MacOS Mojave v10.14.16. (I have not updated it for a while.)

The issue is that out of ~120 GB available system files occupy >70 GB.

Thus, I am quite out of memory. Some space is dedicated to certain apps, photos, documents, or some other files, but it cannot be compared to the system files. Currently, my Mac has become useless, as I cannot store anything there and I cannot even update the OS.

Looking around forums, I found that I can:

• delete unimportant files -- DONE
• delete unimportant apps -- DONE
• clean trash -- DONE
• clean the browser files -- DONE
• manage system files

The last option includes (as I read):

• clean system cache
• clean time machine snapshots and backups (I do not have other devices)
• remove nonessential old update files.

My `System` folder occupies ~10GB. `Library` also occupies ~ 10GB. But I cannot get the rest of the system files. I tried the following:

• I used the free version of CleanMyMac X -- it allowed me to see that I can clean cache and reclaim ~1.5 GB. Nothing about the other system files. So, I deleted this app.
• I went to `./System` or `./Library/Cache` and cleaned cache manually -- Generally, it yielded around 1.5 GB.
• I ran Disk Utility's First Aid -- everything is fine with the disk.
• I checked Time Machine in the System Preferences -- the backup history was empty.
• I found a folder `Macintosh HD/Library/Updates` that occupies ~7.2 GB. -- IS IT SAFE TO DELETE THEM?
• I used the following code in the Terminal:

tmutil listlocalsnapshots
tmutil listlocalsnapshotdates

According to general recommendations, these commands would allow me to get the list of snapshots that I could delete manually. But `tmutil` gives nothing in the Terminal with `sudo` or without it.

• I tried to update my OS, but failed due to the lack of space (it says I need ~25 GB more to install the update).

My questions are:

1. Is it safe to delete the old updates?
2. regarding my unsuccessful attempts to clean Time Machine (either through the GUI or through the Terminal), should I still bother about it? Means, do I overlook something?
3. Can I reset my Mac and update it again? For example, if I get problems with Windows, I can just reinstall it. Can I do the same on Mac? Because I could update the OS, but I am so much out of memory that the recent system update failed.

I am not a big Mac user, so any suggestions will be appreciated.

[Edited by Moderator]