Hello Dorte and welcome to the Forum
I am sorry to hear about Magni's health issues and it sounds as though he has been through the mill with tests and medications which must be distressing for him and for your family especially when you are still not quite sure what the problem is.
Only a vet is legally able to diagnose and suggest treatment for animal health issues and should have the best means to do so. However, I can offer some information that may be of help to you (although without any personal experience of Giant schnauzers) and possibly a Forum member with experience of the breed may also be able offer some insight.
A few years ago I was lucky enough to be able to attend a seminar by Dr. Jean Dodds where she discussed Canine thyroid issues and highlighted the fact that many vet practices carry out a limited and generalised test for hypothyroidism which does not take into account variables such as age, breed etc. which may therefore fail to diagnose an issue with some dogs. I don't know how extensive the tests were for Magni or whether his symptoms justified looking further into this but I believe you can have a blood test carried out via Dr Dodds clinic which does give a much more in-depth analysis re possible thyroid issues:
https://www.hemopet.org/hemolife-diagno ... d-testing/
https://www.hemopet.org/wp-content/uplo ... Dosing.pdf
There is a condition which is known in a number of Giant breeds including Giant Schnauzers called osteochodritis dissecans which is a kind of arthritic condition that can affect shoulder, knee or hips. The Fitzpatrick website link below states:
"The clinical signs are variable and depend on the joint affected and the size of the cartilage defect. The most common signs include lameness, stiffness, joint pain or swelling, reluctance to exercise or play, or general depression."
Both the links below give a bit more information:
https://vcahospitals.com/know-your-pet/ ... cd-in-dogs
https://www.fitzpatrickreferrals.co.uk/ ... ecans-ocd/
If a dog is in pain when they move, it is likely they will become more subdued and less inclined to exercise. Many older dogs do develop some kind of arthritis in their later years. We have a small dog who is over 12 years old now and who was diagnosed by a vet a year ago with the beginnings of arthritis and recommended some supplements which have been excellent for him - his chiropractor was really impressed wth how fit he is on his last visit. We are in the UK but just to give you an idea of the kind of supplements he has. He has a daily tablet of Yumove
, a daily tablet including turmeric and a drop of CBD oil with added turmeric. Turmeric is a good anti-inflammatory and our vet at the time recommended it for regular long term use over Metacam which can have serious side effects used long term. Turmeric supplements should include other ingredients so best either prepared by a veterinary herbalist or if you can purchase or make "Golden Paste".
There are some links here which give more information on the above supplements:
https://www.dorwest.com/product/turmeri ... -and-cats/
https://www.dogsnaturallymagazine.com/h ... -for-dogs/
As said above, only a vet can diagnose Magni's condition but I hope the links give you some useful information. Do have a browse through the Board Index topics too as you may find a post or topic on Giant Schnauzers or Health that is useful too.