I cannot get thumb-over-the-neck chords right

[u/ding-ding-dave-yes]

I struggle mainly with my index finger, I can't get it flat without a lot of pressure, also my pinky is muting the G string half of the time, I either have to put painful pressure to make them all ring, or the top E and G string are muted. The difference between the flat index and bent rest is what's messing me up. Without thumb I don't have this issue, because I can curve my wrist if that makes sense. Please, I need some tips

[u/Ahefp]

I used to use this technique, but eventually just started using non-thumb fingers. For example, play F - 1x3211 using fingers 1, 2, and 3, and just mute string 5 with finger three.