a bit hard to tell from the angle, you need to take it from the side.
back rounding could be due to flexibility issues - tight hip flexors?
pressure in the lower could be due to you leaning forward too much - which is probbaly cause by using too narrow a stance. To stay more upright widen the stance and angle feet outwards.
is this your typical squat workout?
Squat: 135 x 5, 185 x 5, 225 x 5, 245 x 5, 265 x 3
that's why your not progressing. - Not enough work volume
I assume 135 and 185 are more warmups? In that case tonnage here is 3145lbs
if you were to do 8 sets of 3 with 205lbs with 3mins rest - pushing every rep fast - tonnage is 4920lbs. That means more work = progress, and you will be able to move everyone of those reps faster = more tension on muscles.
Sure your using lighter weights and your ego may take a bruising, but in time you will doing way more.
this is training the modern way - not just for strnegth but mass too