Bruce is 100% right, but that's also one of the reasons how/why they can be difficult. I had a sop with no hook that I eventually had to sell because I was developing a tendon problem. The easiest fix is to have a curvy.
There are several modified thumb rests that can be purchased, although I have never tried one. A simple but effective help is to use a bit of thick wall plastic tube that fits over the thumb rest. If the right type (thickness and flexibility) it will soften the pressure and alleviate the indent problem.
Embouchure is another problem as the sop is definitely more difficult, especially if you are playing a "vintage" instrument and need to adjust for intonation problems. I'm not familiar with the SML mouthpiece, so don't know its shape. A thicker mouthpiece means that the sides of your lips need to close around and down further to seal the mouthpiece. This can be overly tiring and not even due to excessive playing. Thinner beaks are generally easier, but with time I think you can get used to almost anything. If you have a teacher or someone more experienced on sop around have them check it out to see if there are other mouthpiece problems that tire you by trying to compensate for something like a table that's not quite flat. Bruce and others would know a lot more about potential mouthpiece problems, but we would all just be guessing without someone trying/testing in some way.
Lots of words, probably not much help. Oh well, sending sympathy and best wishes.