First question would be whether you ultimately want to transition to a clean break or just want a lot more time to do other things, because if it's the latter "

First question would be whether you ultimately want to transition to a clean break or just want a lot more time to do other things, because if it's the latter "stepping back to a more advisory role" is perfectly normal and need not be seen by anyone as a negative judgement of the company. Otherwise the tactful explanation for your decision is the other pursuit(s) that you want to focus on for the next year or two, coupled with the suggestion a new CTO more accustomed to the stage your startup is at might actually benefit them .