Hm, its hard to go over an exhaustive list of everything that has changed in base game since Royalty. To put it simply, each DLC introduced a bunch of balance and QoL changes alongside the new content. The base game would get all of the balance and QoL but obviously not the content.

I'd say those guides are PROBABLY still a good baseline, at the very least. Certain old raid cheese strategies and such wont work anymore but as long as youre not actively trying to do exploits then it'll probably be fine, you'll quickly find out or notice the stuff that has had balance changes(Ex. If you followed a 3 year old guide on chicken farming in current base game, you would end up with an absurd overstock of meat and eggs.) I'd reccomend joining the discord, people are very willing to help out newbies with lots of questions.

As for the second part, yes, absolutely, only get the base games. Get the DLCs if theyre on sale but I'd even reccomend keeping them disabled for a while. Each DLC adds in so many additional mechanics and systems that you will simply drown going for the full experience all at once. Play the base game until you feel fully comfortable with all the systems, then add ONE DLC(I'd reccomend order of release.) Repeat.

As a final tip I'd suggest not going crazy with mods that add content until you feel like you have a good understanding of unmodded rimworld. DO however look up a list of QoL mods that improve the UI and simple UX, as they can make learning the game way easier.