Other people have already said it: we got Mughals as a modern era civ, which means "modern "era" encapsulates the entire 17th century as well. I am 90% sure when we get Ottomans also get them in the modern era for this reason - sure it's not 16th century of Suleiman, but Ottomans were still a massive global power in the 17th (and in many aspects they still held well in the 18th - econmically they only started looking really bad once industrial rev started in the West).
Thus, Poland in the modern era is the best, as it includes the diversity of
1) PLC at the peak of its power in the 1600 - 1648 period (including the military achievement Poles are most proud of: being the only ones aside from Mongols to capture and actually hold Moscow for a while

)
2) PLC during enormous drama 1648-67 which is disastrous period but also very famous in culture
3) Limited recovery in the second half of the century, Sobieski's rule, and famous victories over Ottomans being the swan song of PLC power, the last international moment of fame
4) Potentially - all Polish history afterwards, which was very dramatic, sometimes tragic, but also had a ton of cultural achievements and finally the century of succesful modern state building (actually economically speaking the golden age of Poland is not 16th century but literally now, as we've never in history been as close to Western income and productivity as today in AD 2024)
Personally I prefer that over Poland in the exploration era because while happier and more peaceful it was less interesting by comparision. When you add exploration era Poland you get: 1) Golden ages of the 15th and 16th century - but modern design also catches the spirit of it in the first part of 17th, 2) 14th century, which was already overexploited in the civ series, with two games hard focusing on it, and 3) Honestly very unremarkable Polish history before the 14th century (of essentialy medium sized, backwards, not very distinctive Latin kingdom of only local importance)
Also,
putting Poland in the modern era would allow it to face Prussia, Russian empire, most likely Ottoman empire, and potentially also Austrian and Swedish empires, as they too would get into modern era if it encompasses 17th and 18th centuries. By comparision, who the hell would we fight in the exploration era? Normans and Spain?