Update: Me and Dad changed the power steering fluid today. We emptied the reservoir first with the engine off, disconnected the lines underneath the reservoir and then cleaned the inside of it with brake cleaner. We had to clean it multiple times though, as the inside of it was very dirty. We cleaned the reservoir the best we could and then used our compressor to dry the inside of it before refitting.
Then we connected the lines back onto the reservoir and filled it back up with power steering fluid (CHF 11S). Then jacked the front of the car up, turned the steering lock-to-lock several times, emptied the reservoir with a turkey baster into a container when the car was off, filled it up again with fluid, started the car and repeated the process until the fluid was green. The reservoir always had fluid in it before starting the car, so the steering system was never running dry. Checked the level one last time and made sure the area was clean of any accidental spillage, then closed the bonnet. Cap is securely on the reservoir too.
Its safe to say the steering definitely feels lighter than it was (even lighter than yesterday when I picked it up from the suspension work being done) and the power steering fluid is now green as it should be (as opposed to black!). I wouldn't go as far as saying the steering is as light as it should be compared to another Volvo with much less miles though, but its a massive improvement. The power steering pump is still working anyway and hopefully it'll be happy with the new fluid for a while longer. Its worth noting, I couldn't hear any unusual whining or unwanted noises from the pump and there's no leaks either.
It'll probably need a new power steering pump down the line, but its okay for now. Happy with the result.