Saddle height looks good on the video (I wouldn't raise it, especially if you feel just one side of your butt hurt, because that indicates an imbalance of some sort which are usually made worse by raising the saddle).
How do you feel when you try to raise your hands from the bars while pedalling on the trainer? If you feel excessively like falling forward and having to pedal much faster, try sliding the saddle back a bit and try again and see how you feel. Saddle setback can influence balance and weight distribution on the bike pretty dramatically. A shorter stem might be of use, especially if you change the setback some. Experiment with setback on the trainer and see how it feels when you slightly increase it.
As for lower back pain, all my little aches and pains when it comes to long all day rides disappeared after I started lifting weights. Best thing I ever did for bike comfort.
Last edited by Branko D; 07-22-21 at 05:02 AM.