OK, so you have some spinal flexion at the bottom AKA butt wink. First of all, you are squatting as deep as possible, which isn't bad if you can do it safely but it's not really necessary. Try cutting your squat a few inches higher, before your back starts to round. Another thing you can try is to forcefully attempt to extend/arch your back as you approach depth. In addition to keeping your back from rounding, it should also have the effect of making you catch the stretch reflex at a higher point. And finally, you can also do some pause squats after your heavier squat sets to get used to having a neutral (or slightly arched) back in the bottom position. Go light on those for now, heavy paused squats with a possibly rounded back sound like a dangerous idea and you need to work on technique more than anything.
Another thing that comes to mind, which I can't really tell from watching your video, is that you might not be bracing properly. Chris Duffin has a few videos explaining how to brace, look them up because I can't explain them better than him.
Also, what exactly are you training for? Do you plan to compete in powerlifting or are you training for some other sport? If you intend to get into powerlifting then wearing a belt (but not necessarily on everything) could be a good idea and will also help you learn to brace more effectively. If you are training for another sport then a belt probably isn't much use except for real heavy sets because you don't wear a belt in any other sport.