Hi Qray,
The information you have provided creates an image for me in the following way: two cubes are sitting on top of each other and inside of the two sits your model.
The first thing I would do, is to limit any segment number that will not increase the quality of your result. Given that both cubes are with their Y axis vertically, I would set this axis to one (or two) for the segments. This takes a lot of work away from your system, and will not lower anything.
The Segment amount will not really improve the edge that you create. The main idea is to look at your model without the Phong Tag if you are up to make a mold. To my knowledge the smoothing of the phong tag will not evaluated in mold creations. Please check that with your service or options, perhaps I’m not aware of an option here.
Based on that, no Phong Tag, you need to find a sweet spot where the points are nicely represent your target. Again the Cube segments will not really increase the quality—f you go over the amount of the model.
I have here a little clip (one minute, download it—if you like—for personal use only of course.
https://www.amazon.com/clouddrive/share/wBsy0NBQeAyXHlJYddgoNwvDr-g3rreqy62zV-Sy8Vs?ref_=cd_share_link_copy
Try to find here a segment density that represents the model, as said above, and I try to shocase this here. Especially the Y segments and how they have no influence at all. If you set the cubes to 100x100x1 or 100x100x100 the amount segment is roughly three times higher in the second case (20,400 vs 60,000).
The Optimize should be smaller than any distances to visible points, otherwise it will take away precision. IT is a function to eliminate double points and make your model “water-tight”. IF too large, again, you might get around the seem (connect point of the two cubes) problems.
Single object take the work out to connect and optimize later.
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Alternative, make a copy of your file, and cut it (plane) in the middle. Separate it, extrude the new edge and build from there the cube. It might be more precise than any other option.
All the best