How Can I Permanently Delete All Personal Files On My Computer

I’m trying to delete all the personal data on a laptop. I did read a lot of things about this on makeuseof.com but I don’t think I fully understood it. Here is what I did:

I copied all the files I want to keep to an external harddisk and then deleted those files(on the laptop) with EgisTec Shredder. Then, I used CCleaner and wiped the free space(49 GB) with 1 pass. Then, I restored the laptop to factory settings. Then I ran CCleaner and wiped the empty disk space(110 GB this time, because of factory settings) again with 3 passes this time. To make sure that no data is recoverable, I downloaded Recuva and it found a lot of(like 17.000) files. I don’t know if it found the files I copied to external hdd, because I can’t go through that much file.

So, what should I do? I tried ‘securely delete checked’ on some files Recuva found but I guess it doesn’t really delete or overwrite them. They’re still there and they look ‘recoverable’. By the way, most of the files’ name are “ZZZ….”, I guess it means that those files are overwritten by CCleaner. Does it mean that if I(or someone) try to recover those ZZ files, it won’t work because it’s already overwritten?

Did I do something wrong with CCleaner settings and it didn’t wiped the entire free space? Or am I using Recuva completely wrong? Like, should I uncheck ‘Deep Scan’?

Sorry for the long message, I tried to explain everything I did. Thanks for any help.