Their number one geopolitical problem is being surrounded by too many enemies which means they need a strong army and can't really focus on the navy. Still, a Sweden or Denmark style navy should be doable assuming they can afford it (which OTL no one was really interested barring one period of time). Ideally a reformed PLC should be that but on steroids since they'd have several times more population than Scandinavia combined.
In addition to Denmark, it could get ocean access through the Black Sea since it's pretty much obligatory for a Great Power PLC to subdue the Yedisan (and probably the rest of the Wild Fields) instead of anyone else. As for how they deal with the Turkish Straits, maybe they'd cut a deal with the Ottomans in exchange for assistance against Austria and leaving the Khanate of Crimea around on the peninsula itself?
Although IMO reformed Poland-Lithuania in its late 16th or most of its 17th century borders is a great power already assuming it industrialises at the same pace as, say, Italy and annexes Prussia and Courland. A more centralised government means they can actually use their manpower and build strong local institutions. While it's not the behemoth OTL Russia was in the 19th century, it's definitely at least Austria-Hungary levels of powerful.
Colonies shouldn't be too hard since TTL Poland-Lithuania should be able to own at least one or two Caribbean islands and can maybe play off Western European powers to hold onto it/get it returned after treaties. Maybe sponser lots of privateering in the Caribbean to boost local naval strength there? Same thing with Africa, OTL Courland grabbed both Tobago and coastal Gambia for a short period of time, so if the Commonwealth throws their weight behind it they should be able to grab a sugar island and a few "slave castles" which can translate into a small colonial empire in both areas.
Early 20th century Poland-Lithuania would have a sizable population, a small colonial empire (an African colony, a Caribbean island or two, and maybe a concession in China), and a global reach economically and militarily. It likely has enough power to keep Germany disunited, or united under favourable terms (gaining Upper Silesia would be very helpful). The main enemy is still Russia which at this point would be stronger than ever and probably allied with one or more of Poland-Lithuania's enemies.
It would need to somehow annex the Rus lands so it doesn't have an Eastern border. Then it can become a Great Power on the same basis as Russia did: shed loads of manpower. It pretty much has nothing else going for it.
Russia is obviously a major threat, but they can keep Russia occupied in plenty of other ways besides totally destroying the Russians. Even though this means they'd need strong diplomacy with the Turks (meaning no Yedisan) since the only other real ally they'd get is Sweden. Sweden is probably their best ally
in theory since they can solve overlapping claims on Livonian claims and they both face a major problem in the form of Russia.
Where does PLC get a modern army from? It is a backwards, agrarian economy dominated by nobility that fight any attempt of centralization, with a massive land area and long, hard to defend borders.
Land borders are a mix of rivers and swamps on the east and a decent amount of mountains on the west, so the defensive situation isn't quite as bad as it looks. And the size of the country is another advantage defensively since they can trade land for time to prepare.
It's just there needs to be that centralisation and not letting foreigners interfere in domestic politics. Have it end up something like the Habsburg monarchy or at worst, the Habsburg monarchy of the late 19th century.
