Piano playing should be effortless. If there is any strain, you are doing something wrong. You do need to get that hyperextension out of your thumb, keep working at that. Maybe by lifting your forearm from the wrist fulcrum. You appear to also have a forward shift which is good but play from the weight of the arm. NEVER press into the keys, let gravity depress the keys and try to play to the point of sound rather than into the keybed. Again, never press into the keybed. Nothing can come of it once the sound is generated. Newton's third law will hurt you.
Abducting the fingers creates an immediate and fatigable strain to the tendons. Try to allow your fingers to relax and return to their natural state between octaves as you slowly practice. You don't want to hold tension in your fingers or static load them.
One of my teachers had me put hand lotion on my hands and then facing the front of the keys, or a table edge, she would have me push my hand against the edge of the table and stretch my fingers so there was no longer a gap between my thumb and index finger. Then she would have me do it with all the other fingers and thumb.
I don't suggest that method but it did work for me. First, my teacher developed major thumb issues so and was laid up with an injury for about two years . . . what did she know. Second, I now know that stretching tendons can create micro tears to the tendon and the tendon sheath resulting in ganglion cysts, inflammation of the tendon or scaring of the tendon. None of those are good.
Frankly, I would not be a slave to notation and if you can't effortlessly play octaves, don't play them.
Keep in mind that your fingers don't play the piano, your arm does and your arm places the fingers. Take a gander at this video. In it, Edna suggests using your pronator muscle to "open up the hand." It is also commonly referred to as "rotation." This is an excellent lesson but the kid is clueless at first. If you stretch, you lose. Throw your hand open rather than stretch it open. Relax between notes. Up is more important than down.