I have experience in similar situation, not sure we are talking about the same project.
I suggested full rewrite and got fired in 3 weeks (actually it was a subcontractor role). I have been considering myself as be really good at presentations and persuading executive people to understand what I am doing and what I will be doing, but the situation was too much for me to take on. They didn't like this unrealistic 3-months roadmap to rewrite the whole thing, which does nothing on their point of view but still needs paying the whole team (even though I was the only one). So I told them we are gradually improving it, and did this full-rewrite underground on my own. It consumed me ~13 hours every day, but I was happy myself and was enjoying the birth of the product. Finally after 10 weeks, I gave up to myself and their frustration.
Regarding your problem, I totally suggest dumping your codebase into a git repo first of all, add some cypress/playwright testing to carefully probe the major functionalities, build ci for these, and start gradually removing old version files. After then, just forget how messy it was, what you thought in the first place, consider this beast as a perfect engineering gift (like linux kernel), then start making small changes then adapt yourself into it.
Guide the team to follow your methodologies to treat the code, and tell the executive team that the legacy codebase looks great but complex enough to move quickly as it was a brand new startup project.
For many Mattermost (or Slack) users, it's a painful exercise to update the profile status, especially when you are leaving your computer. What if screen lock/unlock automatically updates your status?
I've created a simple tool to automate this process. (Works only on focal)
