I'll come back here with an old moto of mine: use the classical guitar seated playing position, this is the best position to release all tensions in the body, but don't use a foot rest as it will twist your spine and ruin everything.
Instead use a Hopf Dynarette on your thigh to perfectly balance your instrument and body, but don't buy any Amazon fake, but the original that you can find at Thomann.
Check here what it is:
And watch Simon Zauels here playing the perfect way to avoid any health problem:
To release the tension of the thumb, use the double bass (again) classical technique, which is using the thumb as a guide for your hand without any pressure on the neck, which is also the classical guitar technique by the way.
Remember that to achieve this, you'll have to play a perfectly balanced instrument with a low action and a perfect set-up.
And don't think about what the others will say seeing you playing like this, but think only about your own comfort.
You'll probably think too that it's only a thumb problem, but everything is intricately linked, talk to a physiotherapist and you'll be amazed by how important your posture is (and not only while playing an instrument).