Pinch zoom might help solve this problem of small unreadable text in pictures and equations. New 6 inch Kobo Aura has pinch zoom. Sony T1, T2, and soon to be released T3 has Pinch zoom. Kobo could have pinch zoom for your HD Aura in future firmware or not. We just have to wait and see. The open source programmers at Kobo Developer's Corner might have something that would help with their Koreader program for Kobo eink readers. Pinch zoom can not show picture details that is not in a low resolution jpeg picture displayed on your reader so it might not help.
https://www.mobileread.com/forums/forumdisplay.php?f=247