The reality is that if you join a startup, you should take your job day to day. You should consider it an adventure. Do not expect it to be organized like a corporate job. There is going to be a lot of GROWING PAINS, for young folks with no real world experience, they don't know better. For someone experienced in the industry, man, is it painful! But with that experience, you have to figure out how to lead, your work is not just to code, the bigger puzzle to solve is how to slowly bring about best practices, and you will get a lot of resistance.
You lead by showing. Don't worry about others, they don't wanna write tests? fine, write your own tests, write extra for others if you can. They don't want to document or use revision control? Do so, one day, someone is going to read the comments in your code and realize that it makes sense to document, or read a process document. It might take time, but they will see the light.
In a startup environment, do not complain! It helps nothing, you must wade through the garbage, that's just the way it is. I'm much older and I know this reality. I work for a big company, it's "boring", it pays great, a lot of startup's court me trying to get me to get on board for half the salary and promise of fun, but nope! I'll work more with less discipline and less pay and no stability. I know this. Should I ever join one, I can assure, I won't cry if they shut down the next day after I joined, that's the gamble. Startups fail more than they succeed.
I did try to flag problems, I added more tests, I proposed improvements and went ahead and implemented them. And here we are. Seeing the light implicates seeing anything at all besides pieces of paper with funny faces on.