Hi Orla
Good to see you here again

Sorry I can't help with your thumb though.
I'm hoping to do Grade 4 in Spring too but haven't been playing scales for a looooonnnng time
My problem is a stiff right shoulder and arm which makes my tone sound horrible. Oh and I stick my right index finger up... and my wrist is too stiff... and.. and... I think we adults have a rough time trying to put all this together the right way up

I'm not trying to steal your post - I just think we adults are also very hard on ourselves... I wish we could share our experiences over coffee somewhere
Hope someone can help you out
Happy violining