Invoicing and DES doubts

I wanted to summarise a few questions I have related to being a micro-entrepreneur.

My micro-entreprise providing prestations libérales, started on the 1st of March 2022. I chose to pay my social contributions quarterly.

I begun having an old company I used to work for in the past, as my first client (EU based) at the beginning of April and the first pay came a few days ago.

That being said, here are my questions:

1) I worked for this EU client I mentioned during April and wrote “April Invoice” as title but on the issue date it’s written “issued on 05/05/2022”. - For the French taxing system should I consider the invoice’s issuing date, paying date or date in which I provided my services? If they consider the issuing date or paying date, my social contributions for April will be 0 EUR as my very first issued invoice and pay as a new micro-entreprise, came in May - Am I right?

2) I’m using SHINE as bank account for my micro-entreprise and I’m making use of its invoicing system. On my first invoice, I only had the option to add “Reverse charge - VAT due by the taxable customer” and no space for “3n franchise de TVA, art 293B CGI”. Is it considered a mistake? Can I do something about it at this point? Should I find a way to add it next time?

3) Everyone keeps on mentioning that I need to make a DES declaration to https://www.douane.gouv.fr but I do not find any guide at all on how to do that - Should the declaration be done monthly even if I chose to pay my social contributions quarterly? could you suggest me a good tutorial online? (I already have TVA intracommunitaure)

Thank you tons in advance.