Generally, I tend to encounter such issues and squealing or grinding brakes and can attribute it to poorly fitted pads, incorrect or missing shims and more importantly, lack of compound between the pads and caliper carrier bracket.
By not applying a compound such as copperslip (which is not so necessary on stainless shims) you would have a corrosion factor with dissimilar metals.
You have done everything exactly as it should be done (and believe me, most of the issues I have encountered with dragging and squealing brakes are when those who fit the brakes do not do this).
Also, a lot of pads (if supplied from a dubious source of origin) may sometimes be slightly larger due to excess paint on the pads, which sometimes needs scraping back to fit correctly and this leads to the pads 'grabbing' and not retracting once applied.
Outside of the pads and calipers, issues such as a faulty brake servo, ABS and other related hydraulic issues it may be worth checking all wheels spin freely when jacked up and see if you can isolate the faulty component.
Also, check the brake hoses are okay and not damaged in any way
Let us know how you get on with it