These are looking great and I think there are a few small tweaks we could make here and there to improve them even further. I'm looking at the from the power-lifting point of view.
- Tighten up the starting position
Your lower back is a bit rounded in your starting stance and you look a bit unbalanced. This can be fixed by pulling yourself into the bar more, as mentioned by Dave Liepmann. Another possibility could be that widening your stance a bit. The deadlift is often thought of as "leg-pressing the floor", don't be afraid to use your legs and sit into the deadlift a bit more. The deadlift is a hip-dominant movement more than a back exercise. A great exercise for working on the position of the floor is Paused Deadlifts. Bring the bar 1-2" off the floor, no more than mid-shin, and hold that for 3 seconds then complete the rep.
While you're standing/sitting right now, bring your shoulder back and down and feel how your lats tighten up. You want to feel that in your starting position. The proper starting position for a powerlifting deadlift is tight; it is not comfortable.
- Directly above the bar
If you watch your first rep from the 225 video, you can see that immediately after you pull the bar travels forward. You want to make sure your shoulder lines up above the bar so that the direction of travel is straight up and down.
- Drive through your whole foot
In the first rep of 255 you can see that at lockout your toes come off the ground. When you're driving the bar, you want your feet to be firmly rooted. When doing so, you want to drive through your whole foot, equally:
Image found here: barbell-strength
Next time your deadlift, think about how those points on your foot feel before you pull the bar off the ground. Additionally, your shoes might be impacting the lift (they look pretty squishy). If you're allowed, you can deadlift in socks. I don't suggest on hardwood though; I've had bad experiences.
- Shrugging at lockout
I can't quite tell but I'll just mention it. There is no need to shrug the bar up at lockout. Back straight, shoulders back, knees locked out is the finish position. You don't need to pull it back any farther than that.
Overall, they look like really good reps. Your hips and shoulders move in tandem which is excellent, a big fault I see often are people doing the deadlift in two parts. Props for learning to hook-grip, you're a braver man than I. 22 and 20 sessions since August is not a lot of volume. I'd be aiming for hitting squats/squat variations and deadlift/deadlift variations twice a week if you'd like to see faster progress. The fifth rep of your 255 lift moved fairly quickly though and I'd estimate that you had another 2-3 in the tank (to absolute failure). If your goal is general strength building, then any progress is great progress. Track your lifts and review your progress over the years, not weeks.
If you are interested in testing a 1RM, it is fun and I encourage it. I would not say you're a novice so you know what not to do. I might guess that you're hovering around the 300 lb mark for a 1 RM. If you wanted to try, I'd suggest something like:
- Warmup: Bar x 10, 95 lb x 5, 145 lb x 3, 185 lb x 1, 225 lb x 1, 255 lb x 1
- 1RM Test: 265 lb x 1, 275 lb x 1, 285 lb x 1, 295 lb x 1, 305 lb x 1, 315 lb x 1
Essentially, starting with the fact that we know you can do 255 lb x 5, let's work up from there in 10 lb increments. Stop when you can't perform the lift without majorly sacrificing form. I also suggest filming because I find that during a very heavy lift I feel like my back is rounding but it isn't. Additionally, this would be instead of your working set (don't perform this after).