This question was asked some time ago but remains on the unanswered queue. I want to echo Nate's comment regarding the Journal of Humanistic Mathematics. I've published two papers there. The first one included plenty of "serious opinions on weighty matters" such as the balance between writing research papers concisely vs welcoming new mathematicians into the field, and our collective habit of obfuscating the thought process that led to a result, leading to the impression that doing math successfully requires an inexplicable stroke of genius. We also opined on the importance of finding ways to share mental imagery with readers/listeners, and finding ways to explain math research to the general public. The second paper included opinions (and data from surveys) about many topics, like the importance of play as part of the research process, the value of expository writing, the importance of sharing personal hardships so that junior people know it's possible to bounce back from setbacks and still have a productive professional career (I have done this myself a few times), improving upon the culture of spreading knowledge via word-of-mouth (especially with an eye towards making math more inclusive, including to people who can't easily attend conferences), and more.

I'm a big fan of the Journal of Humanistic Mathematics and would love to see more people submitting articles there. They certainly seem to welcome (well thought-out, potentially research driven) opinions on matters that many mathematicians care about.