OK, thanks for the detailed reply. My reply is going to be somewhat long, as there is a lot to cover. I just hope I do not forget anything. So please bear with me.
First, yes, when I say a complete backup, I mean a complete backup using software like Time Machine, or a couple of other choices (will get to that soon).
Secondly, what is the capacity of your external drive? And also what brand is it, and how long have you had it?
Third, unless you don't mind your important information (both personal and other) being in the cloud, I would not recommend backing up to the cloud. For me, that is too risky, no matter how many "safeguards" there supposedly are. Backing up to an external drive is good enough, but you need to keep at it (along with doing other things). Also, you might consider getting another external drive, and then you can make 2 backups. Myself I have two external drives (actually SSDs inside nice, slim enclosures), and I back up both of my Macs to both drives (I have each drive partitioned into 3 partitions. Two of the partitions are for the backups of each of my Macs, and the third contains miscellaneous "stuff").
Fourth, regarding disk cleanup/maintenance/repairs from a software perspective, I mean using software to do those tasks (and for disk cleanup, you can also do some).
Fifth, you should download the following free software (I have provided links):
Onyx -
https://www.titanium-software.fr/en/onyx.html (Make sure you get V3.4.2, as you are running OS 10.13.3)
AppCleaner -
https://freemacsoft.net/appcleaner/
GrandPerspective -
http://grandperspectiv.sourceforge.net/
Malwarebytes -
https://www.malwarebytes.com/mac/
Onyx is very similar to CleanMyMac, only better. It has been around for quite some time, has had numerous excellent reviews, and is rock solid. I have been using it form so long that I depend upon it (along with the excellent commercial program Tech Tool Pro).
You actually can use AppCleaner right away, given that you want to remove CleanMyMac. Besides removing the app itself, AppCleaner will also find most other files "connected" with CleanMyMac that are not needed. After you remove it with AppCleaner, it might be good to do a search of CleanMyMac on your internal drive. I don't use Spotlight, preferring to use the excellent freeware program EasyFind. But if you use Spotlight, that's fine.
Sixth, your other third party software seems fine. Nothing nefarious there! (By the way, I use Transmission, which is similar to uTorrent).
Now, before talking about backing up, you had another question as to why disk cleanup/maintenance/repairs, from a software perspective, needs to be done, given that the internal drive will be Erased and Formatted. In actuality, and in preparation for that Erase and Format, you will need to do the following:
A. Run through disk cleanup/maintenance/repairs. The initial tool you'll use for that is Onyx. Depending on how you want to proceed with backing up, Disk Utility will come in handy also.
B. Do a final backup of your "stuff", using backup software.
OK, now about backups. It would be best if you read this thread I started back in November about backups:
https://www.mac-help.com/threads/backups-backups-backups.223959/
It si rather comprehensive, and other folks made some excellent contributions to it. As I mentioned in that thread, I use SuperDuper!, and it works like a charm!
After you read that thread, let us know which backup software you want to use. Then, the final steps of the plan can be completed.