You sound very much like I did especially the neck and shoulders however I have had mine since I was 13!! and I think it’s more Dystonia.
I took one of the Now suppliments with 120mg L-Dopa!! without even realising what it was and I felt the effects in 10mins!! all the stiffness in my legs and hips which had been getting worse over the last 12 months just vanished. I can’t say my neck and shoulder are any better.I took one everyday for a week and I saw a big improvement.
Like you I had various tests all came back clear so I wrote to the consultant explained my reaction to the mucana and he started me on a trial of sinemet.
While I waited for the medication I realised how strong the originals ones were so found some with 50mg L-Dopa and just took one every couple of days which seemed to keep me ticking over.
I have the sinement now 150mg a day and although I feel a lot better in myself it’s not really reducing the muscle tonne (however my legs muscles are massive, so it might take sometime) or it seems to be a lot more slow and steady compared to the rapid results I first saw on the Dopa Mucana.
I recommend reading the book Mucana v Parkinsons by Rafael Gonzalez Maldonado if you want to know more and he even gives you an idea of what dosage you should be taking.
Also if it’s just muscle tightness and no other Parkinsons symptoms maybe ask them about Dopa Responsive Dystonia.