Something I do
I was with my teacher the other day, and while I was playing, he stopped me and asked, "why are you playing your tenths like that?". I pondered it for a moment. "Like what?".
Apparently I have my own little technique for large hand stretches that's a bit unorthodox. At least, my teacher hasn't seen it and after about 10 minutes of "research" I can't find anyone else who does it either. I find it easier to play tenths (and harmonics, but only in high positions) by "unhooking" the first and small fingers (like on a guitar when you play barre chords) and locking the thumb at the heel.
I dunno. Maybe it would be better to shift. Maybe what I'm doing is better---or perhaps only better for me personally. Try it. I'm curious what others think.
That doesn't look so unorthodox to me. To a degree, tenths is just a matter of doing what works for you -- without destroying your hand or wrist, of course. I'll bet the main difference between what you are doing and what your teacher would prefer is probably just the amount of tension you're suffering. If so, relieving that tension should be, for the moment, your only goal.
There is nothing wrong with your hand position. In fact it is the way Paganini would have played 10ths, extending back with the index finger since it eliminates some shifts. When playing in first position after playing in third for instance he would have used the same technique of extending back while leaving the thumb in place and moving it only when necessary.
Looks exactly like what Ruggiero Ricci recommends in his book "Ricci on glissando". That book is one of my summer projects and even if you don't want to go all in with a complete left hand technique overhaul there is something to learn. Like playing a scale in three octaves and play all semitones with the same finger which is practically not a shift. That way you can do three octave scales without shifts.
This is similar to the way I play 10ths. I would carry on with it -- as long as it isn't otherwise causing you problems and tension.