All you need is a guy proficient with a chainsaw. I don't mean a
neighbor with a rusty saw who uses it once a year. I mean a guy who
earns a living with it (or used to). You're up north, so you might be
able to hire a logger or tree trimming guy who's an artist with the saw.

That kind of guy could cut those after the posts are sunk in the ground.
He could do three slots in a post in about 30 seconds and they'd be as
straight as if they were cut at a factory.

--Yeah --- that too if you can find that kind of guy :^}

But, if you use a router, make a jig from plywood or similar material
that can be set on the post and direct the shape of the slot. Not
freehand.