"When I started my second job search, I was worried that the short tenure in the job I was trying to leave would be a turn-off to perspective employers. In reality, however, it wasn’t a big deal- people asked about it, but seemed satisfied with my 20 second summary and then moved on to other things."

Alternatively, you can simply leave your new position off your resume and not talk about it in the interviews at all. There is nothing wrong with saying "My last gig was a software engineer at Stripe. I left there to focus on looking for a better fitting opportunity".

There is no need to have your resume be a complete record of your employment. In fact, such resumes are often less desirable because they are long and don't highlight any specific strengths. Instead, make your resume highlight your most relevant and best accomplishments.

I've only been a professional for 10 years, but I am already condensing my resume, removing descriptions from my first few jobs. I imagine that as time goes on, I will even group jobs together like "2010 - 2015 - Software engineer at Companies A, B, C" to keep my resume a nice, neat one pager, and focus on the more important things I have done recently. Like the OP, I've also quit two jobs shortly after I stared them during my career and those are definitely not on the list - its simply not relevant.