Retaliation against US tariffs is the EU’s only real option.

Option #4: nuke BS intellectual property laws that enable rent seeking US firms.

Cory Doctorow has a policy proposal for Canada or any other countries that want to respond to US tariffs. The tldr; is make it legal to jailbreak your devices, so companies can sell aftermarket kits to do things like install a Canadian (or EU) app store on your iphone and cut out Apple's US 30% cut on all app purchases. Make it so any mechanic anywhere can jailbreak your Tesla or tractor or whatever to enable all your car's paid subscription features permanently and for free, etc.

Strike directly at the big American tech companies that prop up the stock market.