Onyx is an excellent app for routine maintenance, and is free. You can definitely donate to the developer but you aren't required to purchase the app.
Yes, Onyx is excellent, and I have been using it for a number of years. But, it is
imperative to use the correct version. For Mojave, Titanium Software today released V3.6.8. V3.7, which is for Catalina, has
not been released yet:
Titanium Software | Operating system utilities for Mac - OnyX
Generally, you don't need to do much maintenance on your system if you manage your system with some common sense, i.e., leave enough free space on your machine, keep only the applications you need/use, delete the rest. Get applications only from reputable developers. Run a number of ad/malware blocking extensions in your browser (like uBlock Origin, Ghostery, etc.).
Well stated, and so, so accurate! Some folks have "chastised" me for being so meticulous with my cleanup, maintenance, and backup tasks, but I don't care. All of it works well for me.
Just a couple of additional things that I do:
1. Permanently removed deleted EMails. I use Thunderbird, and it is easy to do that.
2. Make sure to do frequent backups to at least one external device. Myself, I use SuperDuper!, and make weekly backups for each of my Macs to two separate external SSDs.
3. At times, run the excellent malware/adware detection program Malwarebyes. It is free, and works well. I also have ClamXAv, which is more "powerful" than Malwarebytes, as it can also detects viruses.
