First, that machine is not very old at all.
Secondly, you
DO NOT WANT MacKeeper! That is one of the most perilous programs you can use on a Mac. And, CleanMyMac is not much better. There are numerous ways you can perform all the same tasks (and other things) with some effort on your part, and with good, proven software (both free and not free).
Now, the first thing you need to do is to do "real" backups. You can either do it for free, via Time Machine, or you can spend some money to purchase a product that will do a bootable backup/clone of your entire system. Time Machine is OK, but it is not such a simple process to recover from a Disaster with Time Machine. Yes, it can be done, but it does take some work. Myself, I use the excellent program called SuperDuper! to make a bootable backup/clone of my "system". I use it for both of my Macs, and it works very, very well. In fact, I have twice recovered from a minor disaster using its' recovery feature, and I was back in business quickly. The other excellent one is Carbon Copy Cloner. The primary difference is that Carbon Copy Cloner also backs up the hidden Recovery HD partition, whereas SuperDuper! does not (also, SuperDuper! is less expensive). But, that is not a concern for me, as 1) I already have the two excellent disk maintenance/repair programs TechTool Pro and Disk Warrior (along with having the necessary El Capitan OS files), and thus I do not need what is on the Recovery HD partition, and 2) I have an easy way to re-create that Recovery HD partition.
Here are the links for SuperDuper! and Carbon Copy Cloner:
SuperDuper! -
http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
Carbon Copy Cloner -
https://bombich.com/
So, before starting on attempting anything "serious", you need to do backups.
As for #8, I already mentioned TechTool Pro and Disk Warrior. If you want to get serious about disk maintenance/repairs, you should invest in one (or both) of those. As I said, I have both, and every Saturday, I take the time to 1) do some disk cleanup using another excellent freeware program, Onyx:
http://www.titanium.free.fr/onyx.html
2) run numerous features of TechTool Pro, and 3) finally run SuperDuper!. I do this for both of my Macs, and I do two SuperDuper! backups to two different external drives for both of my Macs. Recently, someone on here asked me about that process. Here is the link where I answered that person:
https://www.mac-help.com/threads/a-question-for-honestone.221623/
Finally, here are the links for TechTool Pro and Disk Warrior:
TechTool Pro -
https://www.micromat.com/products/techtool-pro
Disk Warrior -
http://www.alsoft.com/DiskWarrior/
As you can see, TechTool Pro is less expensive, and it has some more tasks it can do. But, I have read (and it also happened to me once) that Disk Warrior can sometimes do "miraculous things" that other products can't do.
All 5 of those products (SuperDuper!, Carbon Copy Cloner, Onyx, TechTool Pro, and Disk Warrior) have been around for quite a long time, and they are proven, reliable programs which have also had (and continue to have) numerous, excellent reviews.