Brake squeal is produced by the pad composition. The ability of the caliper pistons to retract after a brake application prevents squeals when not braking. Shims are a way of mitigating a squeal, but they produce a softer pedal feel. Many high quality pads have a form of a shim bonded to them, to introduce a bit of squish and reduce squeal, while trying to keep a high, hard, pedal.
So either the pads you bought are low quality, or at least not good for the Volvo caliper/rotor assembly, or the calipers are a bit tight. From your description it sounds like both. Squeals going around a corner are another sign the caliper has one or more tight pistons.
For what it is worth I found PBR pads excellent on 240's and all my customers liked them, but PBR pads on the 740 floating Girling calipers were real squealers. Volvo pads went into 740's ( but dusty, wore quickly, and were more expensive) and no complaints.
No amount of goo on the shims is going to fix it.