I would be taking a good look at your nutrition, and particularly your protein intake. A few years after I got into riding and subsequently lower level racing, I developed a real passion and got heavily into training. My program was sensible with hard and easy days and a total day off the bike, but my legs were constantly sore. Not bad sore, but very much like you describe. I started delving into the nutrition side of things and my first trial was protein increase via a whey protein isolate straight after a ride. My soreness disappeared totally aside from some soreness after hard rides. So the moral there is my protein was down.
Now that was many years ago now and I’ve tried a lot of things since then. I now don’t get to ride nearly as much as I should, but just a couple of years ago I was heavily immersed into the track sprinting side of things. Hard intense workouts coupled with minimal road rides but they were still there. I was no longer taking any supplements as I had my diet sorted and I seemed to be doing really well. So in saying that, get your diet sorted and make sure you’re getting enough of the things that your body needs to do the things that you want to do. WPI helped me through that discovery period and I did end up taking it for many years after. Get a small batch and give it a try and see what happens, or try some of the protein drinks that are around nowadays. Just check you’re getting at least 20g or protein in a serve. It’s easy and relatively cheap to try. From there I hope that you get some relief, if not it may be worth doing some further medical investigation