Hey Jason, thanks for being patient!
Talked about this a bit with the team - the general consensus was that this is more of a general workflow problem rather than a Graphite workflow problem. You're right that most of the advice was geared towards lowering time to review for features A/B/C. Given that your reviews take longer than ideal, stacking unrelated features does seem like a way to get all your changes in faster (but not necessarily the correct way).
It's great to hear that all of you guys are Graphite users - I'm wondering if there are ways you can use Graphite to help bring more visibility to high-pri PRs. Specific PR inbox filters for these features so the SW team can see them immediately? Graphite Slack notifications so they know when they're tagged as a reviewer on a high pri PR?