A few thoughts.
So long as you're catching pacing issues in the writing phase, you should be able to fix them so no need to worry too much! Revision passes exist for a reason- once you have something written out once, you can review it, see how it reads, and see if you need to adjust it at all. If you're sensing that the pacing is too fast then that's a cue to slow it down a bit. I'll defer to the previous response with some good ideas on how to do that 
This is a good talking point, and something that's good to learn about comics as well! You're right that if you have too many panels of just talking heads yammering back and forth then that can start to become boring for the reader. But that doesn't mean that you need to cut the conversations short- it instead means that you should look for more ways to make the panels more interesting to look at even if it does contain a lot of dialogue~
For example, you can play around with camera angles and try to get the best framing for each line to properly convey the emotion, or play it up for dramatic effect. Maybe you have a low shot looking up of a character burying their face in their arms so their conversational partner can't see, but we see their scrunched up sad face and tears starting to fall down. Or maybe you pull the camera back a ways to show both characters at once and their physical locations and turn it into a mid-conversation establishing shot. Maybe you take a moment to point the camera at other important objects around the room, such as a photo sitting on the hearth or a particular spectator in the crowd of a stadium battle, or etc. etc.
If you're newer to comics these techniques may not be so obvious, but it's something that you'll learn and improve upon over time!
This too is a good thing to consider and it sounds like you're on the right track! I'm not sure exactly how your writing/planning process works currently, but what you're describing here is something that a lot of comic artists do called "thumbnailing" or "storyboarding"- basically laying out a whole scene or chapter in a really rough/simple way up front to check to see that your pacing, paneling, and page compositions are working out
It helps to get the written ideas out on paper in visual form so you can get a sense for how the comic actually lays out. Sometimes it takes more panels than you would think to convey a certain part of a story, and sometimes it takes fewer. The key is to do them really rough and sketchy at first- they don't have to be clean drawings, you can just run through with like stick figures or really simplified scribbles or whatever. This way if you have to change things around you aren't losing too much work.
Then once you've finalized your layout and made sure that a scene or chapter has a proper flow and pacing to it and all the panels work on the pages, at that point you can go in to do clean sketches and all that.
One last thing I'll talk about since it didn't fit in anywhere above is to not be afraid to split episodes up if you find that they become too long while editing for better pacing. I'm currently grappling with this in my own series- I have a few episodes that I've had to split up a few times because as I started laying them out in panels, I realized that the panel count was getting way out of hand lol.
For example, the one I'm working on currently: It started out as a single episode when I was initially writing it out. I've been aiming for like ~15-20 panels an episode. I ended up splitting it in half already because I could tell it would be way too long unless I omitted a bunch of panels. So I have the first part finished at like 20-30 panels I think. Now that I've been working on the 2nd part... I'm going to split it in half again. My current script for this one is reaching towards 40 panels, so... yeah xD What originally started out as 1 episode that would have been way too fast paced is now 3 after making adjustments along the way.
