Have you seen this page?
http://www.vivquarry.com/wkshts/linkwd.html 
BTW, I don't think that linking words are too simple and that phrases are necessarily better and more complex. It's a good idea to have a variety that you can use so you don't repeat yourself, so use therefore/thus/hence, etc. Also, as a teacher, I often feel that learners try to make their connections a bit too obvious as a way of fishing for marks. Experimenting with position in the sentence can be another very effective way of using them. Most learners automatically use 'however' at the start of a sentence, but moving it to a later position can be effective and less obvious.
