nice pulling, your back rounding isn’t actually all that bad and what’s happening is totally natural when the weight gets heavy. All you need to do is make a few small changes, and do some tecnhique work for a month and you’ll be laughing.
A few things:
first, you try to arch your back as you set up, then as soon as you start pulling on the bar you lose it. This tells me a few things, first, you’re leading the lift with your back which isn’t optimal, and your stomach isn’t tight enough.
To start, you need to start doing all your rep work in the 80% range, and 3-4 rep range. This is just heavy enough to be challenging, and light enough that you can keep tight form. That’s the key, all you need is repitions and you’ll learn how to keep that back flat.
The next thing is you need to learn to use your legs to get the bar past your knees. I’d strongly advise you to start your deadlift workouts with dead lifts to the knees, with 75-80% for 5 sets of 3, then deadliest for 3x3 @ 80%, you should notice a big change in your form just off of that alone.
Now, your low back: This may sound weird, but stop trying to arch, it’s too damn hard anyways. Instead, learn how to keep a neutral spine. This is a great series of videos on how to do that, as well as how to train a hollow body, which is almost the same thing, you just don’t round over. Everything else is good though.
Lastly, after you get past your knee, you’re pulling BACK… Instead i need you to think of standing tall, and bringing your hips forward, this activates the glutes and your lockout will be greatly improved.
Take care bud