I'm happy to report I got a good-looking-no-bumps panorama out of Hugin. See attached.
The only bad news is I'm not really sure how I did it.
Good to know it worked out as intended.
From the looks of it, you've selected a cylindrical projection, which makes sense with such a wide field of view. That does mean that only a centrally placed horizon will be straight, and that's what you seem to have done, vertically centered on the horizon.
You can also achieve that automatically, even when your image tiles have more sky (or more land) than land (or more sky), by placing some horizontal control point pairs in each image tile, if it's easy to see that horizon like in your images. By adding those horizontal control point pairs in each individual image tile, you'll automatically force the image to a higher or lower than center image, and subsequent cropping will take care of the rest. It may cause empty areas in the outermost corners, and/or the center, so one might be required to take some additional images for those areas if the scene is predominantly sky or land.
Other projection methods
offer other opportunities, but also challenges. Not all projections are available in every Pano-stitching application, some only offer a few.