Got back into this after putting it down for a while.
What does the expansion do for you that mods don't already add in?
Hmm...
Royal Titles, which are kind of like a lite version of dorf nobles, are not replicated by any mod that I'm aware of. Patches after the initial release watered down the impacts a lot, though, unless you actively want to deal with them or have bad luck. (Only certain types of colonists will become nobly snooty now.) I'd say they're kind of 'eh' - some interesting perks and challenges to titles, but nothing huge to an average game of Rimworld.
Psychic Powers is another version of Rimworld of Magic, and I think I prefer RoM if only for having the option to autocast certain spells. (Thankfully you can have both running simultaneously.) Though supposedly the upcoming next version of Rimworld does an overhaul of sorts to the psychic system. There's another psychic system mod that was released after Royalty (and doesn't require it), though I haven't tried it yet.
Quests... well, there are plenty of other quest mods out there, though this one does have a better UI system for it. Not sure if the quest UI upgrade was folded into the base game, though. I do like the quest types added, though they can be a little hit or miss in terms of risk v. reward. (That said, just about every patch since release has been tweaking those values, and it is getting better over time.)
Mech Clusters are a different enough raid/threat type that I'm not sure if there is an equivalent mod. One could argue that boiled down they're just extreme variants of sieging raiders, I guess? I like them, personally.
Imperial Tech is probably another version of Glittertech, though I never used that mod. Bladelinks (i.e. DNA-coded weapons) are unique, and I think the techprint system is also unique. It's fine, but there are plenty of other high-tech mods out there already.
New Music - well, there are lot of music mods out there, but none by the original composer.
Not a feature, but one that is still kind of significant - having a high-tech faction part of the base game. Some are fine with it, some think it ruins the original theme. Matter of personal taste, but (at least without modding) one can't selectively add the other features and not the new faction.
I'd say it's worth it on the whole, but I'm lucky enough to not have to worry about affording $20. I
think most of the major mods out there don't require Royalty, though, so I wouldn't call it critical.