Perhaps this might be useful for PCR tests? Also for a flight departing by tomorrow, Aug 4th you can take a PCR test upon arrival (this provision ends by tomorrow!).
https://newyork.consulfrance.org/cor...e-faire-tester
Otherwise I find it very difficult to find concrete information regarding EU citizen transiting through other EU country (like overnight layover) but it overall appears that's not a real problem. Most countries though require some for of quarantine (very specific to the country, unfortunately) but you can likely get a swift PCR test at the airport (for instance in Prague, result in 12 hrs or expressly in 3 hrs) and in Czech Rep., PCR test done in Czechia within 84 hrs of arrival waives any quarantine requirement.
It's quite difficult to get a swift turnaround on a PCR test in US right now given the amount of tests made daily, unfortunately.