You can patch your Kobo firmware the my line 10 patch would do what you want.
https://www.mobileread.com/forums/sh...d.php?t=323419
Note I’m linking to the current firmware patch thread please make sure you read it and ensure your firmware matches. There are patches for older firmware as well.
My line 24 would also work and I think give you more granular control but as of the last time I looked we’re waiting on this patch. But my line 10 should get the line spacing down a bit more.
There may also be limitations with how close the lines can get with that font