why kathiran and not jiddan?
Does this mean I very much like reading, or I like reading large quantities of material? If not both, how would you say the other one in Arabic?
It means you really enjoy reading.
Could I write القرائة instead of القراءة? Would it make any difference?
ئ is 'i, not 'a. It therefore is incorrect to write القرائة.
So can i say "shukran kuthiron"?
To say "thanks a lot," yes you can say that (but note it's "kathiran")