I am a graduate student in a theoretical STEM field, studying under a professor who is relatively well-known and respected in my field. My experience working under them has been...not the best. It's nothing too crazy or egregious - it's not as though they're abusive or academically dishonest - but they are often nearly impossible to reach (emails are almost always ignored and getting them to read paper drafts is often a 4-6 month long endeavor) and they would generally provide minimal supervision on projects, which has led to a rather difficult PhD process. I am about to apply for post-doctoral research positions, and my question is, how should I go about answering the (presumably natural) question during interviews of how research with my PhD advisor went? There have been good aspects of our relationship, and I intend on bringing those up as well, but an honest answer to that question would involve these more negative aspects. I think I'm capable of answering the question honestly without sounding bitter or resentful (my advisor was quite busy, so I imagine that contributed to a lot of the problems), but I'm just wondering to what extent I should temper my response further, or if I should speak candidly about my PhD experience.
I don't expect you will be asked about how your research went with your advisor, rather you will be asked about your research. It's yours, not your advisors. Excuses won't make your research sound better than it is.
I think it's fine to be forward-looking regarding past experiences that were less than positive, and say you are seeking some specific attribute or opportunity in your next role, but don't dwell on what was missing or suboptimal in the past, only on what you want to do to grow and accomplish more.
It sounds like you've had a lot of opportunity to work independently as a PhD student - that's an asset you can promote without throwing your advisor under the bus. Your next advisor is likely to find it easier relate to your last one than to you directly, and they won't want to be under any heavy vehicles themselves.