How can I tell if I'm being too picky as a referee?
In my humble opinion, it looks like you are the ideal reviewer actually! You give a lot of advice to improve the paper, and this directly benefits the authors and the journal. And apparently you give very precise advice, which is much more useful and actionable than general or vague remarks.

My main concern as a reviewer is to be fair in my final recommendation. As long as your meticulousness doesn't lead you to reject potentially good papers, you are doing a good job as a reviewer. However the question of whether you are spending too much time on it depends on your priorities, it's important to weigh the benefits and costs for yourself before you accept. It's perfectly acceptable to refuse a review from time to time in order to maintain the level of quality for the ones you accept.