Annie my girls (1 year old) get layer pellets (red label I believe it is) constant supply, most days I throw them a handful of free range scratch when I go in there and what ever household scraps we have, they go out in the back yard to farage most days. With free ranging they take what they need, you don't need to worry about what they eat, as they know what they need, but when you are mixing up your own foods for them they will eat that over the pellets and disrupt the ratios. I would keep your special mix to once a week or even less, and make sure you give it in the afternoon after they've already had a day of foraging and pellets, with scratch seeds make sure they have to work for it, throw it on the ground rather than in a feeder. It doesn't sound like they are unhealthy at all but given they are around middle aged it's best to watch their intake a little more closely
