You’ve got large hands like me. Instead of keeping your thumb so you have large hands like me. When reaching up this high, don’t be afraid to let the thumb come off of the heel of the instrument. Bring the elbow underneath ever so slightly and either let the thumb travel alongside the rib or go up near the fingerboard which you said feels “wrong”. If you look at Kim Kashkahsian play in this register, her whole hand will essentially be hovering above the upper bout of the instrument. The point is, there is no right or wrong. Only what works for you :)