maybe but how does this will clean the hdd?
The majority of files that you probably want to wipe (and others that you probably would like to keep) are located in your user account folder. Some more might be in your /Library folder where system wide settings are usually stored.
plus my admin user is my main user
That makes things a little more complicated. If you had a standard user account, it would have been a click of a button to delete that and make a new one. It was long time before Apple ID and SIP that I was fiddling around with the root account to get an initial admin account exchanged with a new one including having the same user ID (UID).
If you don't optimise for disk space, you could just create a new admin account and don't touch the initial one. Then maybe install the combo update, make an Etrecheck to wipe third party plug-ins that were installed system wide and you should have a quite clean working system with a flooded and trashed initial account on it, that doesn't bother you.
If you don't tolerate the initial admin account, then a
factory reset is maybe something to think about. I reckon that part:
restart in single user mode (Command-s)
Code:
mount -uw /
rm /var/db/dslocal/nodes/Default/users/<shortname>.plist
rm -r /Users/<shortname>
rm /var/db/.AppleSetupDone
reboot
After reboot, you just choose the same short and long user name that you already have and your initial admin account should get overwritten. CAUTION: Some apps that store license keys or other needed files in your user account could refuse to work. I can't test that ATM, so I don't know if it really works, but...
...as the others pointed out, you made a full working bootable backup first.
