Trade might be a good thing in general, but this comment is a bit reductionist with respect to the role of US Big Tech in suppressing and controlling EU tech

The EU seems to have no problem legislating against and penalizing US Big Tech in return. IMO the European attitude towards entrepreneurship and startups does more harm to EU tech than "US Big Tech" ever would or could

I was recently talking to a friend in Spain about a startup idea and he said no way could he join a startup (in a senior role) because if it failed his career would never recover.

AFAICT it really is the attitude pushing so much innovation out of the EU and not the legal environment much less the competition. The newer members like Hungary and Romania seem to have a lot less of this problem (but more of other problems like stable business environment and predictable rule of law).

What kind of harm, i.e. which stats to look at to compare the different approaches?

That's an entirely empty threat. Our trade deficit is larger than the GDP of half the countries on earth combined. What are we going to do with what we purchase in Europe - switch to buying it from China?

The US started this already by imposing tariffs on European goods like steel.

Plenty of US people like and make open source software. You don't need to be worried.