How come I am being constantly prompted to update to the buggy 6.0 instead of the rectified 6.0.1?
It depends upon the manufacturer pushing the update. For many number of devices you would end up in same situation even if the first release of a platform upgrade was buggy. Let's take it this way:
Assume that these are the Android versions - build numbers for your device:
Android 5.1.1: build LJKRTY;
Android 6.0.: build MOUIPV;
Android 6.0.1: build MOUIPW
When you would switch from build LJKRTY to MOUIPV a lot of the files required for Android 6.0 would be upgraded. Since the manufacturer came to know about a couple of bugs in the recent release, they decided to only patch the bugs and release the patch as an update with build MOUIPW instead of packing and releasing the whole platform upgrade again.
Now there is no point in applying a patch if at the first place both the vulnerable and relevant code isn't there. You may also not be able to apply such a patch manually since manufacturer can require an update to check whether the currently installed build is compatible.
So over and all, switch from Android 5.1.1 to 6.0. and then to 6.0.1. That's the way to go.
By the way, it is possible that the update for 6.0.1 may be a full blown platform upgrade but your software updater is stuck at 6.0 simply because it is silly. You can attempt to clear its data (if it is a separate app), force stop it (or reboot) and attempt to check if it still requires to download Android 6.0. I won't keep hope here.