How to get consistent font size and layout when converting to kepub (from epub)?

[Thorned Rose]

I have a Kobo H2O2 (and previously an H2O) and prefer to use kepub (I use the stats a lot) I have issues with weird line breaks after converting to kepub if I use font rescaling (it's always set to 12pt).

See here for side by side of original epub (left) and converted kepub (right) using Calibre's E-book Viewer:

Note: For copyright, book text has been pixelated.

The font size is set to 16px in both. When I sideload these to my Kobo, the font size in the kepub will be what I expect but the epub font size will look giant. (Calibre's E-book Viewer shows the kepub as having larger text despite, as above, the font being set to 16px. The font size on my kobo looks like the text on the left, just with the messed up layout on the right).

Obviously if I "Disable font size rescaling" I get really inconsistent fonts across books which is especially problematic if I'm reading several books at once - not very practical to have to change the font options every time I switch between books. The fonts also tend to be much larger although not anywhere near as huge as a straight epub.

How can I get a consistent kepub font size suitable for my Kobo without the weird line breaks on things like titles, especially when said titles contain things like images (as above screenshot)? TIA!

EDIT: Essentially I want to retain the original epub formatting/layout, just refactor the font size so that it's not ginormous and has the additional kepub stats.