The most important thing is to turn slower than you think you need to, so that you have enough time to lookahead.
Something you may or may not have thought of: if you want to, you can just solve centers, then stop and scramble again, and so on, to get more practice specifically with centers. Other than that, there isn't much I can say about centers without seeing a video example of how you solve them.
For edges, I can't give you anything regarding avg edge pairing because I am terrible at it, but I do use freeslice, so I'll give you a tip. It can be very useful for lookahead's sake if you don't just take the first piece you see and then try to find the pieces of the same color. What I do is, after solving centers, then I solve all four black (or white if using a black cube) edge groups first, and then I solve all four yellow edge groups, leaving the four groups that don't have standing-out colors for last. So while solving the last two centers, I can look around the cube and specifically take notice of any black edge pieces, making it very easy to transition from centers into edges. Then when I am on the last black edge group, I start looking at all the yellow edges I see, looking for matches. By solving in this way, you don't have to think about as many pieces, which can help reduce pauses.