Well, I thought I only had blurry vision my entire life. I was only diagnosed with double vision/MG when I was 41. I was misdiagnosed with lazy eye at age 10. I've had MG since around birth. That's why a N-O is so important to see.

VGCC is the test they do for LEMS.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4360501/

MGL1 - Overview: Myasthenia Gravis (MG)/Lambert-Eaton Syndrome (LES) Evaluation

Congenital myasthenic syndromes are prevalent in Northern Europeans (UK included, in spite of Brexit!). There are blood tests readily available there for that. As you can see from these sites, there are a lot of diseases that affect the muscles. However, MG/LEMS/CMS are all about fatigable muscle weakness. But each has its own unique features.

Neuromuscular Syndromes

Congenital Weakness

Myasthenia Gravis: Differential Diagnosis

The LRP4 test hasn't been available that long, and the same company who does the MuSK does that one.

Huperzine A acts like Mestinon does, as a cholinesterase inhibitor. They both inhibit the enzyme Acetylcholinesterase. I have not tried that supplement. I use Mestinon (don't use both!). People with a normal amount of acetylcholine getting to their muscles can have symptoms of an overdose and cause muscles to become weaker.

Even natural substances can cause serious side effects. Diagnose first, objectively experiment later!

A SFEMG can be positive in LEMS and a CMS. In some of the CMSs, people can have too much acetylcholine. And in some of those, the ocular muscle are spared. How long have you had your symptoms? You say decades. But have you had symptoms since birth? The CMSs are genetic, and symptoms can be present since birth.

Oxford University does a great deal of research on MG and CMSs. I can't say that I would recommend seeing anyone there though.

I hope your docs can pinpoint exactly what is going on. That's important! If you have a CMS, for example, steroids wouldn't help, since they are not autoimmune diseases.

Annie