Let's assume that leaving right now is not an option. Recognizing that efforts from the lower ranks will have limited upside, what can I possibly do outside of gritting my teeth as I continue to suffer with this awful software?
Or start to manage the software development, if you think you could do better. Keep a spreadsheet of the bugs/feature requests. Send emails asking for dates, do QA and test releases before they hit the client.
Or stage a mutiny and displace the bosses.
But anyway it looks like you can't do anything about the situation, because you're not the development boss. Sometimes it's better to accept that there things you can't control, and life goes on.