I’ve tried thumb position, but i think the way one holds a guitar almost horizontally is not conducive to that position (certainly doable on the A and D strings). I also don’t know if I’m holding a guitar properly, but i’ve pulled my strap (hilariously) high so that the guitar is sitting really high on my chest. This lets me play at the neck where the spacing feels closer to a cello which allows for easier extensions as well.
I’ve had to resort to 1st finger at thumb position and finger 2, 3, 4 at extended position instead of T, 1, 2, 3. If i remember my cello training correctly, thumb position didn’t use much 4th finger or 4th extensions (please correct if my memory is wrong).
edit: fixed confusing finger numbers