If the event is in a month, I think it's a mistake to cram a lot of changes into the next couple weeks. Tweak, maybe, but don't try to cram a lot of change in during these couple weeks. I can recall going to a dressage show a week after going to a clinic and having the clinician change absolutely EVERYTHING and going to the show and thinking, 'I...AM...GOING...To...DIE'. Met a local trainer at the show and she had just done the same thing, so we were yelling at each other, 'I can't halt!' and 'I can't bend!' and LOAO. It's good to have more than a couple weeks to make peace with all those changes, LOL.
After the show, in dressage, I do think you need to drop your stirrups, your knee is up on the knee roll, almost ahead of it at times. You also tend to turn your right toe way out and kind of brace with your legs instead of sinking down into the saddle.
Your horse needs to bend and be more supple so that he can be less resistant through the canter transitions...BUT...I MEAN...SERIOUSLY...the stiffness kind of looks like how he balances, if you went crazy bending excessively, he'd probably find it very hard to balance. That is a very downhill built horse and you do an absolutely incredible job of keeping him together and balanced, all the time, in all the phases. Just Wow. Very good.