Other answers have addressed the issues with no written contract, back pay being owed, etc., and I have no expertise in that area. I would like to add a point of view related to the work: You could potentially leave your company and continue working on the project as a contractor, and this might flip the power dynamic completely.
You mention a "client" ...
If you are in a country with decent employment laws then the next time your salary is late, walk straight out the door, don't look back, hire a lawyer to get your wages out of the company, plus the lawyers fee ofcause and pass any emails or letters you receive from the company directly to the lawyer.
I've been where you are.
Five years or so into my first job out of university, I found myself the sole developer on several simultaneous projects.
I was working insane hours far beyond my (out of date) contract in order to try and keep on top of everything, I was stressed and suffering from anxiety in a big way, I wasn't getting overtime pay or even any ...
I would leave as soon as you have secured a new job. It is a really bad sign that they pay you late and haven't given you a written contract. And if you were promised paid overtime and they haven't paid that either, that too is really bad. Things are likely to get worse, so leave as soon as you can. Your employer needs to do more to retain you than rely ...
Two issues. Your taxes and quitting.
...it's been more than 5 months of overtime money that has not been paid
and there are no benefits whatsoever, sometimes the salary can be
And I did not sign any contract when I was told that I was a permanent
employee, only verbal statements and congratulations.
If you are in the United ...
may I resign in the middle of this project?
The answer to this question is always "Yes"
There is never a good time to resign, it will always be an inconvenience of one kind or another. However, that inconvenience is not your problem; your problem is that your employment arrangement is no longer convenient for you. The project status is your employer's ...
In this case, feel absolutely free
Loyalty demands a certain amount of reciprocity, i.e. the employer must treat you reasonably. Being overworked, having lousy project management (getting that far behind is the evidence of that), not getting paid for overtime, sometimes not getting paid until late, and being unable to sleep all each individually qualify as ...