HI! I’m no rainbow expert, and have few rainbow photos of my own I actually like but here a few quick thoughts of my own:
If you can find some foreground elements that can lead the eye to the rainbow, it can sometimes help. Also,
In regards to focus, rainbows are themselves blurry, which makes it tricky. I find that if I focus on something rather distant, such as a tree or hill, and stop down a little bit, the rainbow tends to come out in focus.
I looked around quickly to find some rainbow shots in my LR catalogue that I know I have, but couldn’t find any off the cuff. (note to self: use keywords) The only pictures I’ve taken that I’ve been happy with, and indeed the only pictures I’ve ever seen of rainbows that I find truly compelling have been images with a human element. I believe this is due to the reason God created the rules of nature that cause them. God brings them about as a reminder of the Noahic Covenant found Genesis 9.13 to remind of sin, judgement, grace, and redemption. Experiment around with having people as foreground for the rainbow, would be my key suggestion.