A couple thoughts on how I think about pacing, in case they're helpful!!
For my most recent chapter, instead of just thumbnailing or scripting from the beginning to see where it goes, I would kind of break the concept for a chapter down to smaller and smaller chunks. So, I'll start with "Chapter 3 is the one where they get to a big city, get separated, meet Xira, and have a run in with the cultist's soldiers." Most of my future chapters are defined this way, maybe at most a vague list of Things That'll Happen In This One -- I prefer giving myself some wiggle room to develop the details of future chapters so that I don't get bored!
Anyway, I take that and I break that down into a list of all the scenes that need to happen ("they arrive at the city, Jonan gets them through security, then they make a plan to meet at the church if anyone gets lost," etc etc)
At this point I can kinda see if that series of events feels interesting, or if there are gaps in logic, or scenes I hadn't really thought about, or if things feel like they're taking too long, before sitting down to draw a bunch of thumbnails.
After that, I break that list of scenes down into a bullet point or sentence for What Happens on each individual page. (Page 1: they crash land outside the city. Page 2: conversation about the guy they're going to meet. Page 3: they learn about the security outside the gate, etc etc) Since I do a weekly comic, this is important for me to do so that I can make sure that each page has SOMETHING happening on it, and it's easy for me to avoid things like "Page 12: they continue running, Page 13: they run some more, Page 14: they finally arrive" where a scene is going to feel way too slow with individual updates.
So by the time I sit down to do An Actual Script or Actual Thumbnails (I do both, but which one I do first varies), I've already had a chance to see whether the pacing is working, and I'll make the thumbnails and script for each page based on the pacing I've already decided on -- so if I know "they have to get through the forest and arrive at the town on this page" then I can't get caught up on the details there -- I know I have to move quickly to get that entire moment to fit on one page.
Jesse Hamm shared a different method for a similar idea on twitter, which I've been using to help me get a handle on my next chapter. Highlights:
I don't have strict page counts for my chapters, but giving myself a rough allotment "if this chapter is around 50 pages, then this important scene should get about 12 pages" seems to sometimes be a helpful starting point for breaking down the scenes and figuring out which moments need to be condensed and which moments need time to build up.