I plan on taking it quite seriously, and I want to make it a long series. I even want to make it good enough so I can receive patreon donations in exchange for chapters in advance (eventually). Anyway, my main platform will be Royalroad. But other than that, should I post it on other sites as well? Like webnovel.com, tapas etc...? If so, which sites do you recommend? Thanks for any advice!
Yes, you should release it to as many places as you can. Just remember that if you need to edit the story after publishing you'll need to update each site. It's a lot of work if you post unedited material.
Not an expert on this subject, but if you're posting it on Webnovel/Qidian, it might turn exclusive & you'll need to take it down from other websites (remember the "drama" when they started their own apps?)
Scribble Hub, RRL - sites like those where you can post your work without strings, obligations or contracts. Or just self-host and aggressively advertise your novel on forums and subreddits where it’s relevant.
What do you want from your webnovel? Are you putting it up for fun and to improve your writing? Scribblehub is probably the best place for you to put it up. Do you want people to pick up your story and follow it, giving critiques both helpful and not? Royalroad is the best place for you to put it up. Do you want to develop it into an additional income stream? I have no clue how to help with this one, though I believe that it relies heavily on consistency, quality, and luck. But if your desire is for as many people to read your novel as possible, then publish it on as many sites that allow for free reading, and don't have a restriction on publishing on other sites as you can, like Westeller said.