First, what Mac OS are you running? I have read about issues with High Sierra and Fusion Drives (that issue is (supposedly) going to be fixed with the next OS, Mojave, which will be released soon).
Secondly, have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You actually can do a good amount of cleanup on your own. One place to look is in your Downloads folder. Also, there is the issues of deleted EMails not actually being removed from your machine. And, there are some excellent programs available (both free and commercial) that can help you with those tasks. Here are some free ones:
Onyx - Available from here:
https://www.titanium-software.fr/en/onyx.html
Rock Solid, has been around a long, long time, and has had many positive reviews. I use it religiously (along with a commercial program entitled Tech Tool Pro). I would not be without either of them. In fact, I will not upgrade to a new Mac OS until there are compatible versions of both of them for the new OS (along with 4 other critical programs that I have). Note that there are different versions of Onyx available, for each different Mac OS. You would need to get the correct version for the Mac OS you are using.
AppCleaner - Available from here:
https://freemacsoft.net/appcleaner/
Another stellar performer! As you can see via that link, it will get rid of most stuff associated with an application that you wish to delete. Like Onyx, note that there are different versions for different versions of the Mac OS. You would need to get the correct version for the Mac OS you are using.
GrandPerspective - Available from here:
http://grandperspectiv.sourceforge.net/
Nifty graphical way to see which files are on your drive. Good way to identify large ones that you might not need.
Malwarebytes - Available from here:
https://www.malwarebytes.com/mac/
Good to use if you see adware and/or suspect you might have a virus. It also is good to run it every so often, even if nothing is wrong.
ClamXAv - Available from here:
https://www.clamxav.com/
More extensive than Malwarebytes in terms of finding viruses. You can use it in demo mode, and it still has just about full functionality.
Then, you should consider using either SuperDuper! or Carbon Copy Cloner to make a bootable backup:
SuperDuper! -
https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
Carbon Copy Cloner -
https://bombich.com/
You can use either them in "demo/trial" mode, although I believe you can use SuperDuper! in trial mode for more than 30 days. Using either of them makes recovery much, much easier. And you will have way more control over the process.
For commercial products, I use Tech Tool Pro (along with Onyx) on a weekly basis to keep both of my Macs "lean, mean, and clean". While Disk Utility is fine, it is not as extensive as Tech Tool Pro.
Third, for any third party software you use, are you keeping up to date with them?