Our backlog is run by whoever shouts loudest

Sprint planning is a ritual we do but do not really follow. The CTO walks in on Monday, points at three things that caught his attention over the weekend, and that becomes the sprint. My carefully prioritised backlog might as well not exist. The rest of the team has stopped contributing ideas because they have learned it will not matter. I need a way to fix this that does not require me to go to war with the CTO.

Background

SaaS company, 45 people, Series A. CTO is also a co-founder so this is not a line management issue I can escalate.

Already tried

OKRs (ignored when convenient). Sprint goals written on a whiteboard (overridden). Tried raising it in a retrospective and was told we are being too process-heavy.