Take a few other things into account. If you look at .raw file sizes, its not simple multiplication for height map sizes. Sure when you take the data created from players living in the map digging etc, the file size will increase, but I think at that point you need to account for 2 other things, overall performance and actual player usage.
Like you, Ive made literally thousands of these and released more than I can count, and the one thing Ive noticed in almost every single case since the beginning is this. Players eyes for big maps are much larger than their stomachs. Players will generally only tear up and really use about 30% of a map, they may explore and cherry pick the rest, but you could almost set your watch to how predictable this is.
The larger the group, the larger the area, but not dramatically so, still ends up being around 1/3 of 6-10k map. I think even you figure what the average player save/meta data per map, and then take into account that 4, 4k maps are equal file size of 1 8k, I think you'll probably end up with better performance and same or less save space, but with multiple smaller maps than can be swapped in and out from workshop as singles or map sets.
From the perspective of good map making in 7d2d and aesthetics, I know from making maps, esp hand made maps, once you get 10K and larger, you either have to import POI sets to fill all that space, or you just end up with duplicates and large samey cities. I understand the natural player love for open unexplored space and the possibilities it puts before them, but I really feel like this can be achieved by loading from 4k to 4k map, with 2k and 6k mixed in for things like 2k boss maps and 6k multi play maps.
For the space of a single 16k height map raw, you get 16, 4k maps. If i split that same 16k height map up into 16 playable maps that players could traverse in every direction via teleport, there is no question youll get much better performance, and definitely higher height map utilization per 4k section. By loading 5 or so maps in at a time, then optionally adding or removing individual maps, you really full use of the total height maps and more space for individual biomes to naturally fit and border other biomes with better transitions. This is all academic, just some ideas and thoughts.
I can see it going something like this down the line and i think they could pretty easily gear something like this to rwg, even cooler you import a giant custom map and it cuts it into 4k maps.
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