Long-term freelancing, or I would call it B2B contract employee, is also the only way to work remotelly for abroad company.

Unfortunatelly EU has no means of cross-border employment yet other than establishing local subsidiary company. It is unlikely to happen if they "employ" a couple of people in various foreign countries. Company setup procedures and employment laws vary by country, salaries may vary by currency. It is just too complex at small scale.

Then the simpler solution is to use intermediary local contractors agency or just directly pay Euro invoices of long term freelancers.

It is legal but e.g. German companies prefer not to keep such contracts longer than 2 years. Why? That is another story...