Sorry to read your post. These "Mystery illnesses" have a whole different level of concern for you. And we in our modern world expect to find answers: as @celia16 says, there sometimes just aren't.
But we must not be complacent. Well done you, for hunting down various disciplines in trying to sort out this scenario.
The one thing that jumps out is that "really bad cold". Have you had a Covid test. I believe at this point a blood test may be the only way to determine if you have had Covid. Perhaps another manifestation of "long-covid"?
The wonderful thing that you point out is this: "slow, small improvements". Yea, we want those symptoms to be gone, yesterday. But consistent, positive changes in the right direction is pure gold. You may already have found the key to eventually getting back to yourself.
Our own experience with this was my son, when 15, became very ill, a "mystery". At one point was diagnosis with a brain tumour wrapped around the hypothalamus"... with just a few months to live. ..... ! There was no tumour.
The final medical answer was: "we may never know what was wrong. but he is making very small gains. We hope this continues". His recovery took 3 years.
Today he is a tall, handsome 46 year old - and his mom's joy.
I so wish for you the same future. You are to be encouraged to hunt for answers and treatments. If you are in the States, why not go to a Mayo Clinic? Use every possible advantage to find answers.
Remember that for all the amazing advances in our medical 'modern' world, we have discovered so much more that remains to be discovered. Stick with your physio and celebrate each new improvement, no matter how small.
All the best. Let us know how your journey progresses.