I've ordered a single replacement key off of Syntaur, but I've read some things online suggesting that you can't replace single keys on this board, that the entire keybed needs to be replaced.

I don't recommend ordering parts until you know what parts you need.

Unfortunately, I cannot seem to find much helpful info or repair vidoes on this older machine online anywhere.

Why didn't you just call Syntaur and ask them using the Contact Info on their web page?
https://syntaur.com/contact.html

I just called them and they answered that question within a couple of minutes.

I also found the service manual available for free online if you want to do the replacement yourself and Yamaha service centers can do that job also.

I have a MO6 and fixed a single key that was "sticking up" before the sale. Although I cannot directly answer your question - I can say that this single key seemed easy enough to service without touching the rest of the keys.

If you want I can open it up and take out a key and replace it and take some pictures.

BTW: Yamaha also sells these repair manuals so if they are available free then these are not legitimate sources (nor necessarily would all paid options be legitimate).

As an update, I just opened up my MO6 and was able to take keys out easily one at a time. I see no issue with this. Behind the keys is a rubber strip that is there to prevent the keys from being removed when using the keyboard normally. Take note of the orientation of the strip. It's not rocket science - one way makes sense and the other doesn't (mechanically).

I can provide pictures if you want.

Also, to revise my comment about single key serviceability without "touching" (removing) other keys - let me say that I believe the black keys need the surrounding white key(s) removed so that the black key to be removed can slide forward during the removal process. Otherwise, all of the white keys do come out individually without having to touch any other key.

I don't see a reason why you would replace all of them unless they were all bad. We're talking about the physical condition of the plastics of the keys (and assuming everything else is OK).