the Spanish tax authorities would treat the USA LLC as a Spanish company. This would result in a significant amount of paperwork.
Told you so.
Perhaps I should take the risk and proceed with my initial plan, registering as an "autonomo" in Spain and invoicing my USA LLC.
As mentioned before, this comes with multiple risk:
1. Your company would have to register in Spain (as you confirmed with the CPA) and follow the regulations for Spanish companies (CIT, VAT, accounting, ...)
2. You probably can't just invoice 100% of the profits of a Spanish company
3. With only one client, you could probably be considered an employee of the company, so the company would have to pay social security contributions for you
4. The US won't allow you to just invoice 100% of the profits
There's a good chance you'll fly under the radar, but you are breaking multiple rules in two countries.
If you are caught, you may just get a slap on the wrist because you didn't actually try to evade taxes, or you may find yourself in serious trouble. I'm not sure, but I'd expect the legal expenses to get high fast, especially when you have to hire a lawyer to represent you in the US.
Ultimately, I would fulfill all my tax obligations in Spain, as I am not attempting to evade taxes; I simply wish to operate under the umbrella of my established USA LLC.
Have you looked into changing the taxation of your LLC from corporation to disregarded entity, as several people in this thread suggested?
If
@TheCryptoAnt 's information is correct (that Hacienda would treat you as autonomo in that case), then that could be a good solution.
What are your thoughts on this matter? Thank you!
I think you should find a good lawyer that can come up with a suggestion. You should especially check your original question about VAT.
Maybe you can set up a Spanish branch of your US LLC or simply a Spanish company. Then use the Spanish company to invoice both European clients and the US company.
Then you would at least reduce the risk for 1.-3. above.
But in general, I would keep trying to find a Spanish CPA who has experience with the US tax code as well.