Squealing noise from brakes

I have a 2001 E46 330CI automatic cabrio in the UK. I changed the brake discs and pads on all four wheels this last weekend and put on new tyres with rims. I properly greased the back of the pads and inner part of disk. I have been driving since Monday without any issue. The car feels a little more hesitant to go and I feel the power is there, but there seems to be a tad bit more resistance more than normal. I put this off to there being brand new tyres, maybe extra friction and they just need to be broken in.

However, yesterday I started to hear the highest pitch squeak from my brakes. I initially thought a wheel bolt was coming loose but I checked and they were all tight. Every-time I break the sound goes away, but when I drive under 20mp the sound sequentially sounds as I drive. It’s so odd. When I brake, it stops, instead I sometimes get brake squeal. Why is this happening?

EDIT

So, after work today I removed all the brake pads and greased them up nicely on the back and the contact points on the calliper. The front left brake pad was seared on the corners so I think maybe it wasn’t sitting right. I made everything sit right and took the car for a spin. The noise mostly disappeared other than the occasional and very intermittent squeak which at least doesn’t sound dis-ordinary considering they were initially sounding like a squealing pig.

But I suspect they’re only quiet because they’re freshly greased and 2-3 days from now they’ll start squeaking again. Is this normal for brake pads? Also after driving at a high speed and braking harshly to polymerise the pads, it would appear only the front left and back left tyre smoked. The rest were only warm. Again, suspicious?