So, it'll depend on what you mean by chapter releases.
If you are planning on doing each release as a standalone, yeah you can just build each chapter separately.
Fortunately, you are using Renpy, so there are ways to 'segregate' your content into chapters, applying patches to previous chapters to add later chapters.
Some people get intimidated by this, but once they've added a mod to a Renpy game the first time or two, it's a fairly straightforward process. You can go about this a couple of ways, but here's my suggestion for the moment:
Divide your game files into chapters (as an example, DMD likes to 'sort' it's pics by day) so you'd simply have an images/Chapter2 folder for chapter 2. Similarly, at the end of your chapter one script, you'd included a jump at the end of your last label script to a suitably named label in a new 'Chapter2.rpy' file (name it whatever you'd like), where for now you'd place a simple label that has your 'to be continued' graphic and a suitable end sequence for your game at that point in the Chapter2.rpy file.
Then just start your chapter 2 script in your version of the Chapter2.rpy file, and give it a similar treatment at the end (jump to label in a Chapter3.rpy file, to be continued). When you have Chapter 2 ready for release, you simply include your Chapter2.rpy file, and the associated images, in a separate .zip, which people will 'merge' with the Chapter 1 files they already have for your game. As a modder, I simply 'duplicate' the directory structure for my mods, and have people drag the /game folder of my mods into the main game folder of the game (so this merges /game with /game).
Note that you'll be overwriting the 'to be continued' Chapter2.rpy file with a new one when they merge the folders when installing the new chapter, and adding the 'to be continued' Chapter3.rpy file at the same time.
You can make installers that do this, so that people are less intimidated as well, but I'm not the guy to ask on that. I've never felt the need to make an installer.
If you have any references to pics from chapter 1 (backgrounds, menu icons, etc.), you simply call those as you've been doing before (in Chapter 1), referencing the appropriate images in Chapter 1. You usually don't have to mess much with screen structures, etc. if you are happy with what you did in Chapter 1, but of course you can simply 'overwrite' screens.rpy (or whatever) with an updated version for Chapter 2, if it's simply a 'quality of life' enhancement that isn't chapter specific. If the screen changes in some fashion, well, you'd simply call out 'HUDScreenCh2' (or some other suitable name for your purposes) in your screen call, to point the game to the updated screen, and of course dismiss the 'old' screen if it is currently being displayed. You can also use tags for this, giving a tag assignment to similar screens, so that only one screen with the same tag will display at a time, dismissing all other screens with the same tag.
This will also allow players to play chapters back to back in the same install, as they are released, and use the same saves throughout the game. Of course, later saves won't work with versions of the games without the relevant chapters, but if a player wanted to backtrack to a previous save from a previous chapter, that shouldn't be a problem, as long as the associated screen files and such are still 'relevant/haven't been drastically changed' (thinking HUD here).
Now, as to how to 'build' a patch through Renpy...
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This explains how to build a patch, but you could use a similar approach to chapters.
So the net result of this is that your existing players can simply download the new chapter, and merge it with their existing install, as you release chapters, hence keeping your download sizes manageable for each chapter. Sure, they will still eventually be donwloading everything (you mentioned 6 GB of content), but it'll be in small increments, hence not as painful. And, more importantly, they won't have to 're-download' the images from previous chapters over and over if they don't want to.
You can also build 'full' releases, combining all of your chapters so far, for players that haven't downloaded your game before. Sure, it'll be a bigger download, but it'll include all of the relevant chapters so far. That way, they don't have to mess with merging folders/using the installer the first time around. You'd still have the 'to be continued in next chapter' label and associated Renpy file at the end of the last 'current' chapter, so that they can pick up with patches from that point forward.
I'm sure others will have interesting suggestions along these lines too, but that's how I've seen a few games handle this now, and it works pretty well. You just need to fall into the pattern of setting up a new script.rpy for each chapter, so that the next chapter can simply overwrite that file when it is released.
Note that you can declare new variables, characters, and associated pictures at the start of the new chapter file (just put them in an init block), or declare them at the point when they are first assigned/used in the story.
One other note, if your script.rpy isn't set up to do this currently, well it's a pretty tiny file, so you can simply overwrite it when Chapter 2 is released, with the new 'chapter structure' at the end of said .rpy file. Renpy compiles all of your labels into a 'chronological' list, it doesn't matter which file it finds the labels in, it'll just add them to the list. So your existing labels can stay as is, it's just the last label that you'd be setting up the 'to be continued' jump to the first Chapter 2 label.
Hope this helps!
