Isn't this 'thick' when used adjectivally rather than the noun for 'thickness'? I'm guessing it literally translates as 'of thickness'?
The noun is 'Trwch' which as you suggest means thickness
Yes, this is used as an adjective to mean 'thick'.
Another deleted incorrect sentence fragment.
It would be used as:-
'Deg centimetr o drwch' = 'Ten cm of thickness' = 'Ten cm thick.