The boost is in the loop of his noise supressor, not the amp - you always want overdrive/boost/gain effects in front of the amp. I guess never say never, but almost without exception you want your gain effects hitting the front of the amp.
Singlecoils don’t sound as “fat” as humbuckers. They’re not supposed to, they have a much more pronounced attack to them, and that’s a large part of their vibe. You may need to tweak your amp settings a little bit but generally I don’t have to touch much when I go from a humbucker guitar to a singlecoil guitar. Try maybe janmming along with a backing track rather than judging them in isolation; it’s just a very different sound that sits in a very different place, but the extremely open dynamics can make them sound “big” in different ways. Ironically, the lower gain is really helpful here too.
You can probably do the saddle swap on your own, if you’re careful and even remotely handy - just measure very carefully (mark, if you can, maybe put some tape down under the saddles and mark the very front of the saddles), then unscrew the old ones from the back and rescrew the new ones back in and just make sure you get them to exactly the same position as the old ones. That said, just because a particular setup worked for Yngwie doesn’t mean you’ll find it’s right for you - you’re used to humbuckers and play through a 5150, Yngwie played singlecoils his whole career and played through old marshalls wound way up.
But, singlecoils sound “fat” in very different ways than humbuckers. You’ll never get that thick low end and darker rounder attack out of them, but the really explosive attack can import its own punchiness, and it’s something I personally really love about singlecoils.