I think the problem with your deadlift is that weight is too light for you. You know it’s too light so you are thinking about doing a heap of reps.
This is problematic. Even elite level deadlifters lose form after 5 reps yet your trying to do 12.
The weight is so light for you that you are puling with your arms. Do that with a max weight and you’re in trouble.
I understand you don’t have any more plates so it’s an issue at the moment but at some stage you need to increase the intensity and lower the reps right down to 5’s and 3’s and heaps of singles.
The deadlift is a technical lift so practicing singles will help nail the form. The set up is vital for a deadlift. If you don’t set it up right you wont lift correctly. When you first put your hands on the bar push your chest out as far as you can. Arch you lumbar as hard as you can and breathe in a huge amount of air and hold it. Push your abs out into the belt (this is what the belt is for).
Before starting the lift pull the slack out of the bar. This means you lean back hard and set your hips into their correct position. Some like to have hips high whilst some keep it low. There really isn’t a one size fits all here. If your hips fly up before you even get the weight off the ground then you’ll know you’re too low but it’s not a big deal and easy to fix. When you start the pull the bar must come up against your shins. Your arms must be dead straight just like levers. Press your heels hard into the ground and thrust your hips forward to finish. If you have trouble staying back on your heels then curl your toes up at the beginning of the lift. This is a good cue for squats as well.
You have the bar going out forward then back. Again I’d say it’s because the weight is too light.
As for what muscles the deadlift uses you’ll find out once you start going heavy. In warm up sets most people feel it in their quads, hamstrings and glutes. Once it starts getting heavy you’ll feel it in your erector spinae, trapezius and even in your lats when you get close to max lifts.
Edit: I just re read what I wrote and watched your deads again and I may have been a bit too picky as your lift really isn’t too bad. I think it’s easy fixed by just keeping the weight closer to your body and scraping the shins and by not using your arms to pull.