So, while I did spend a few months documenting all these issues, the company itself had not made room for engineering-led process improvements. Noticing that, I started spending political capital on trying to fix it, which is incredibly stressful, slow, and unrewarding work. After a few months of this, I finally came to my senses and realized that, this not being my company, I would not accrue any of the benefits of improving their processes. Time to leave.
The lesson, for me, has been to avoid working at companies that are either not product-focused (in which case engineering is just a cost-centre, having extremely low status within the organization), and to avoid stubbornly non-technical founders (i.e. any founder/owner who refuses to care about their engineering department beyond asking "is it done yet?").
Always document product decisions. "Documenting" doesn't have to be a big process, and you don't really need to make polished documentation, but at the very least dump your notes from when decisions are made somewhere permanent and searchable (github comments / jira comments / even slack messages in a non-dm channel count). Your future self will thank you.