Personally, I've alternated between Disco and Chase Freedom with their 5x categories. But for the times in between that, I have been using either my Citi DC or BoA Cash Rewards. I've noticed (in the case of my spending), the difference between 5% vs 3%, is negligible. I'm sure with large spending it'll be more of a difference maker, but for small/minor spending, it's not much of a difference.
I would agree with your approach, convert to the Freedom and use that, also you can get another Discover IT card on the 1 year anniversary of your current Discover card. Most people end up doing that because of the CB Match.