Good question. If you work for a local company income tax is "steep". see: https://en.wikipedia.org/wiki/Taxation_in_Portugal

Portugal has followed the "Nordics" model when it comes to taxes; income is taxed higher but services (healthcare, schools, roads and other infrastructure] is much better than in the US [or most of the rest of the world for that matter!].

The much cheaper lifestyle in Portugal more than makes up for the higher taxes.

I'm not a "tax adviser" and HN is not the place for that "tax efficiency" advice. But you should consider setting up a Ltd Company and paying yourself/co-founders [Tax Free] dividends.

I would [highly] recommend watching "Startup Legal Mechanics" [YCombinator Startup School]: https://youtu.be/BTShgZxiNV8

tldr; start a company [separate legal entity].