I've found that I can output a 6x3 grid of 5000 px image resolution without crashing FTPro on my system. This produces a result, successfully, from a equiangular projection without the river-issues showing up (horizontal lines stretching randomly across my map) at the final resolution; the overall view still shows those issues.

Pasting together the 18 images in PS, now, takes me about 5 minutes.

This process works well for altitude, climate, "other shader", and bump map views, at least.

Rivers do have small gaps where tiles meet (sometimes?).

Intensity though, I put on it's own layer, and find that sometimes I get a dark horizontal line stretching across the top or bottom of a tile, which makes this process fail here. I've taken to saving a single image 9000px in width (the maximum before FTPro crashes out for me).

As well, hold down shift and ctrl. when you actually click on the "multiple images" selection that takes you to the final output screen you posted above. Then you're outputting .bmp images, which don't have the horrible artifacts that the jpg output from FTPro has (it must use a fairly high compression setting for such nasty artifacts).

I feel your frustration.

p.s, a 5000x5000 px output ends up being ~96MB.
p.p.s. a 30000x15000 px final bmp is @#$@#$ huge.