ok try this cause it kinda looks like you are going down and thinking that you have a belt that your posture will be fine
get next to a mirror
as you go down try to keep your lower back tight. personally if i simply try to push my belly out it seems to get tight. now go down. meanwhile keep your chest up. to do this try thinking that you want to hit the wall in front of you using your chin(doesnt make sense much but if you try it you’ll see that your chest stays up). look at the mirror that should be at your side and see if your posture is straight. if not, try to keep your head at the same position while pushing a bit with your feet into the ground, which will rise your butt up a bit. you should feel a stretch in your hamstrings
at this position your head is probably as forward as it can while your butt as back as it can, meanwhile your back being straight. this is what dan john (i think thats his name?) calls the bow .
now make sure your arms are a bit forward from the bar, the bar close to your legs (you already know that)
and you should be good to go. as you get more strength and flexibility im guessing you can lower your butt a bit. im not sure. i see some people deadlifting with head and butt in a horizontal line at start, others with their butt more down and their head more up. im not sure.
btw it seems that you arent keeping your arms out… when you pull it looks like your elbows are bended? is that true? cause that could really hurt your biceps or something. im not sure what but you should keep your arms, grips etc tight, not bended.