What are my options as an Auto-Entrepreneur also working part-time for an unrelated business?

I have been registered as an auto-entrepreneur since April of 2016. I am in the process of writing a proposal for a small french company that is interested in having me develop their business in the US. The work would entail emails, phones contacts and attending exhibitions/conferences and is unrelated to my current activity. Number of hours are not determined at this time my guess would be around 10-20 hrs/week with a fixed commission on each sale in addition to either an hourly wage or per project wage.

Since this is a new venture and flexible in nature I would like to continue my current activity. If this business takes off they would also consider hiring me as a full time employee. Since I have never worked for a company in France or done any freelance work I would like to know what you suggest and what I should be aware of in the future if I am hired on a part-time basis or offered a full-time position.

My concern at this time is not earning all of the monies immediately since I have my other activity but to propose a structure (if it exists) that is the most tax efficient for both of us and the requirements for billing such as time and format and whatever else I may need to know.

I had asked a similar question in 2017 and you had let me know about obtaining a free-lance code from URSAFF and quoted the following below:
In both options, your extra turnover will be declared through your auto-entrepreneur, but at a different social charges rates (both about 22.1% to 22.5%).

Since the initial phase of the project can be done by almost anyone (no special skill set required) I am concerned about charging too much on an hourly basis so that is why I was considering a per project sum.

Also going forward, can you please confirm the time limitations on having an auto-entrepreneur status? Also, if I would need to change what my best option would be if I am working for this company on a part-time basis for the next year or so and if I am allowed to keep my current business if I end up working for them full-time?