I digitized 6000+ slides in early 2018 which were mounted in a variety of cardboard and plastic mounts. The slides dated as far back as the early 50s, with many of that age exhibiting significant curvature.

A number of options were considered, but I couldn't readily find any commercial service that offered comparable quality to what is achievable with modern photo sensor technology. Many are still using scanners that were state of the art in the early to mid 2000s, but are now entirely obsolete. Frankly, I was also unwilling to entrust the sole copies of these photos to Fedex.

Ultimately, the best solution in my case was something similar to this: https://www.pteavstudio.com/forums/topic/24303-digitizing-slides-with-a-kodak-slide-projector/. I bought a cheap USB relay board for about $10 and made a small Python script that would trigger the relay to advance the carousel, then emulate a keypress to trigger a capture in a tethered Lightroom session (tethering is vital for keeping track of as much metadata as possible when you have that many files).

I threw a few layers of diffusion gel between the projector lamp and the slide to ensure even illumination. Make sure to also take a blank frame as a reference for vignette correction. I also experimented with using a hotshoe flash, but using the built-in projector lamp was the only way to get reliable AF, and it produced more consistent exposures in terms of metering and color temperature.

The throughput ended up being pretty good - about 1.5 s per slide when running, most of which was unattended. One issue that cropped up every 2-3 reels was that some of the cardboard mounts would jam in the carousel, which resulted in a mix of duplicate and skipped frames. YMMV, particularly if you have a newer projector.

I'm not sure this is cost-effective unless you already own a 24+ MP camera, and ideally a macro lens (though you can rent or buy/resell one fairly cheaply for this). It's also obviously a lot more work than any commercial service would be. That said, the results are spectacular, and significantly above the specc'ed/showcased performance of any service I found. With a modern (2016ish or later Sony sensor—I used a D600), you can generally out-resolve the slide film media in terms of both sharpness and dynamic range. I think slide film still wins on color depth, but not noticeably.

I would estimate that doing all 6000 slides took around 120 hours of human time. One thing that was particularly difficult and time-consuming was getting the alignment just right and keeping it there between sessions - a geared xy stage (ideally xyz) and a sturdy base or tripod on a hard surface are mandatory. You'll also may want a Giottos blower to get as much dust off the slides as possible unless they're immaculate and you're essentially doing this in a clean room.

For upwards of 20k slides, I would see if you can find a local service that's using one of the commercial SlideSnap rigs or equivalent, though it won't be cheap. I consider drum/flatbed scanning to be an unacceptable loss in quality for this application, and I would really recommend against settling for that.