Excellent progress man!
It is inevitable that your gains will slow, providing you are a natural trainer. When this time comes, instead of switching exercises, in your case i'd say reduce volume. I know it sounds crazy at first, but 12 sets for one bodypart sounds crazy to me! drop down to say 9 sets for large parts and 4-6 for small. I'd also suggest upping calories a little too to ensure you're adding about 0.5-1lb a week.
I personally do about 4 sets for large parts and about 2-3 for smaller. Once you stall on 9 sets, i suggest you reduce volume further and up calories.
If you are progressing in the gym regularly you have NO NEED TO MAKE CHANGES. Why "fix" something that isn't broken? (Clichéd reply, i know)
(the more astute of you will also realise i type "fix". Make of that what you will.)










