How can I convert my Tarkov USD into EUR efficiently?

After completing the Gunsmith quest line, I had about 9000 EUR.

And so had everyone else - including those with no interest in using Peacekeepers offer to trade 50000 EUR for those Sniper skill levels.

So all that was left to to was: flea market all loot for EUR instead of RUB, and buy all equipment for USD. Those who got the Euros for free and have no use for them will pay a significant premium for not having to spend precious Roubles (which, unlike the other currencies, are usable for insurance). Even offers which are not the cheapest according the flea market sorting will be filled quickly.

(answer originally suggested in comment by Sanctus)