Hi!
These are great questions, and I want to take time to answer them because I find them really interesting.
Pacing: I like to think of my series overall as a long-running tv show in the fact that, like an episode of television, every chapter has to both build on the previous one, set up the next one, and accomplish something all at the same time. This is tricky, but it's important to remember that the thing that happens or is accomplished in the chapter doesn't always have to be big. For example, in one of my upcoming chapters, the main cast of characters discuss traveling to a certain planet, and it turns into a debate on some of the actions they took previously, but they still do end up on the planet. It's not exactly major, readers will probably infer that the characters were already pretty much guaranteed to go to that planet anyway, but the fact that they did arrive there and that the characters argued is still a plotbeat that did happen, and it not only helped expand characterization but got the characters to the planet, which means that the chapter contributed not just to characterztion but to plot as well. Now that sort of thing shouldn't be every single chapter, I tend to think most "traveling" chapters are pretty boring myself, but I feel it's fine to have chapters that are a bit slower as long as something does still happen in them, and it makes the chapters where the major plotpoints happen feel even more important by comparison.
IMO, everything is fine as long as the chapter contributes something to the plot. If you find yourself thinking that nothing would change in the narrative if you cut it out, then you're probably doing something wrong.
(Ironically, I feel like this one part of my response went on for far too long...)
Planning: Yes and no. I do let some of my story grow naturally, but I always like to know where it's heading. Before I sit down to write a chapter, I have to have in mind what one thing absolutely needs to happen in it to further the story and let myself fill in some of the blanks from there. I also do bullet point outlines a lot for every twelve or so chapters that are literally just a list of, this happens, then this, then this, and I find that actually really helps me because it's basically just a checklist.
Posting: I used to overthink my schedule a lot, but I always knew that posting every Monday, Wednesday, and Friday was going to work best for me. Tuesday, Thursday, and the weekends are when I have the most free time, which allows me not only to write new chapters but also to adjust and schedule the upcoming ones. The main problem for me starting out (and the main thing you should avoid) is figuring out what time to post. I would worry far too much about what timezone I should target, what hour would get the most traffic, what sort of other stories would I be competing with for that timeslot, etc. I quickly learned that it does not matter at all. I got my most traction when I just picked one consistent time and stuck to it, and even then, most of my returning readers seem to just wait until the end of the week to go through all the chapters for the week, which is completely fair and probably a better overall reading experience.
Motivation: My main motivation is the story itself. Even if no one were reading it, I probably would still write it somewhere, or at least daydream about it, because of how much I've come to love my characters and the universe they inhabit. I'm not delusional enough to say that it's perfect or even good, but I've been having a lot of fun working on it, and it's pretty much become my main hobby. Writing overall is very therapeutic for me, which may seem horrifying to say, given how grim my series and some of the short stories I personally wrote for myself can get, but I do get a lot of catharsis out of a good sob story or seeing a character finally overcome the odds and succeed in their life. I'm an emotional person, and I get very emotionally invested in the things around me, the reason I keep writing is because I am emotionally invested in the journey my characters are on right now, and I will do anything to make that journey as compelling for my readers, even if it means ruining the characters' lives for the sake of the narrative.
I only started posting a few months ago myself, but I wish you luck on your journey, and I not only hope that your series becomes successful, but I sincerely hope that you enjoy writing it as well ♥