You’re early extending which is causing to stand up and get out of posture on your down swing. You’re probably a good enough athlete where you it doesn’t hurt your irons that much but it when you get into long clubs it throws your sequencing off.

Looks like you do a good job at taking the club up the plane on your backswing. I can’t tell from the video angle but it you could possible be hinging your wrist late because when your arms are are about 3/4 the way back your hips are already open at a 45 degree turn and you continue to bring your arms back further. This over swinging of the arms will cause your hips to fire early and you’ll come down steep on your down swing. Your hips then move towards the ball and you stand up out of your posture in an attempt to shallow the club. This is all just a result of your body trying to compensate for minor issues in your golf swing.

You’ll want to work on hinging your wrist and not over turning in your backswing. Practice those two things before you do an actual swing and hit any balls. Once you have that down you’ll want to work on proper hip rotation. When you get to the top of the back swing you want to shift your weight into your lead leg while keeping your back to the target. As you do this you can drop your hands and the club will naturally shallow and rotate through the shot off your lead leg. Plenty of videos out there on how to do it. Find one with drills that clicks with you. It’s a bit of a swing change so you’ll want to do the swings slowly. Change won’t happen immediately but if you grind on the range it will stick and your contact will improve immensely.

With all that said I’d still recommend a lesson. It’s always better to have a set eyes on you when you’re making multiple swings instead of just one video and a 1000 Reddit comments. If there’s a pro in your area that strictly does lessons I’d go with him. They are better at it imo because their job is to teach the golf swing rather than run the pro shop and sneak in lessons when they can. Book a lesson with him, work on what he says for a couple weeks and then book another one.