I’ve used Bay Area Circuits before and as I recall the quality of the boards was decent. I don’t remember their pricing, but they do have an online quoting feature (as one would expect from any reputable manufacturer, and even some of the disreputable ones). They do have an assembly service, but I’ve never used it.
You can also check out PCB Shopper to get quotes on different board houses world-wide. Just ignore the ones outside the US if domestic manufacturing is a necessity (for example: government grant projects). Note, PCB Shopper won’t show you ALL POSSIBLE manufacturing houses, only the ones that PCB Shopper lists. I think only 4 of the places they list are US based. (Based on a quick and dirty check of prices for a 2"x3" board, BAC was the most expensive of the 4 listed US based board houses… Probably why I only used them once…)
Oh, I can’t speak for other board houses, but I do know that OSH Park does accept KiCad board files, AFAIK up to v5.1.x. I’m pretty sure they do internal cutouts (check their design guidelines for the minimum milling bit size), and they announced a little over a year ago that they now officially support plated slots (useful for some footprints).