Power racks and squat racks should usually be equipped with safety bars.

At first I thought he was gonna back squat it, then I saw him put it on his shoulders and thought 'wow, he's gonna front squat that', then he bloody pressed it and my mind was blown.

Not warming up is a recipe for disaster. You take a frozen slab of meat, you throw it on the ground, chances are it'll snap. Now, you let another piece of meat defrost, i.e. you warm it up, then throw it on the ground, it's not going to snap or crack or anything.

And nobody is asking you to put 300kg on the bar the first day you step into the gym.

Are you able to bend forwards/backwards/sideways without pain? Lie flat on the floor and try to lift one leg, keeping it straight. Do the same with the other and then try both legs. If you feel any pain or a tingling sensation in your back or down your legs, stop immediately and if that's the case, you'd best get to a chiropractor as soon as you can.

I can't say anything definitely as I'm not qualified, I'm merely trying to troubleshoot your problem judging from my own experience. But what you can do for now is try not to bend over to pick anything up and be conscious of the way you're sitting as well as the length of time you're sitting for, ie don't sit for too long. Then get checked up at a chiropractic centre as soon as you can.

As much as you can while maintaining good form within the rep scheme of your routine.

From the side so you can get an accurate view of depth and if you have any rounding in your back.

More like I'm nowhere near you two. You'd both wipe the floor with me on deadlifts.

Anyway, what Mike said, form looks solid and depth on the squats is decent.

Wearing a belt or not comes down to the individual. Obviously you shouldn't need to wear one on your warm up sets or for small isolation movements. Personally I use a belt on nearly all compound lifts I do because I can push my midsection out into the belt which gives me a tighter core.

Everything looks good! If I were to nitpick, I'd say try not to hyperextend the neck so much on deadlifts, a more neutral position would be ideal. And squat depth, but you've already spotted that so you know what to do!

Perhaps switch to push presses for a couple of weeks to give yourself a feel for handling heavier weights. Then return to regular OHPs and restart your progression on them.

Form looks good. Only thing I can point out is you seem to lose tightness at the bottom, hence there's a little extra movement. For this, I'd say to remember to take a deep breath before each rep and brace your core, which would feel like pushing your stomach against your abs.

To be fair, there was minimal movement. I was really just nitpicking. But yes, bracing the core will generally better the quality of your reps.

Stop at the point your lower back begins rounding, in your case the knees. On top of that, work on your hamstring mobility outside the gym and slowly progress to reach lower as you get more flexible.