A 12 x 5 will be fine. If you can't get it to slow down with that then something is wrong.

Try flying it at a high angle of attack to initially slow it down and then put the nose down to maintain flying speed on the final approach.

Also, make sure your throttle linkage has zero play. A lot of times the idle speed is inconsistent because there is play in the pushrod that doesn't bring it back to the same position.

Also, if you can use expo on your throttle, try setting it up so that you have a lot of expo at low throttle. In other words, half-stick = 1/4 - 1/3 throttle. That will give you finer control over low throttle settings.

I doubt they make a 4" pitch prop at those diameters, but I could be wrong. If they do, you might want to order a couple.