Actually, P-L came pretty close. Very high personal, political and religious freedom. Very proud of it they were, too. They called it their "Golden Freedom."
Unfortunately, this freedom was solely for the szlachta, or nobility, who were perhaps 10% of the population. A major part of their "freedom" consisting of freedom to tyrannize over those who were not szlachta.
Much as in ancient Sparta or Rome, where being a citizen meant you could stomp all over those who weren't. Or the pre-war US South, where freedom and whiteness constituted a kind of rank in society.
The Golden Freedom of the szlachta of course meant they were also incapable of combining, even with each other, much less with their subject peoples, to resist subverson and/or invasion by better-organized, though less free, neighbors.
While I think you have a point about aristocratic republics, I think "imperialistic" doesn't quite fit in this case -- not in the sense of classical imperialism anyway. The original Lithuanian state (if I remember correctly) was largely an East Slavic country. It was joined to Poland in marriage, and (as Poland was more populous and developed) Belorussian, Ukrainian, and Lithuanian landowning elites gradually became "Polonized" without much difficulty. Much of the talk about ethnic conflict and "subject peoples" -- at least in the early days -- had more to do with class, with resentment about the burdens landowners imposed -- than with nationalism. There were similarities to the situations in Ireland, Bohemia, and elsewhere, but the situation didn't entirely correspond to classical models of imperialism.
I do agree that it would have been very difficult for a nation run like the old Rzeczpospolita to survive, but (as in the case of Russia) one can ask whether things were necessarily fated to develop as they did or whether history might have taken another course given different contingencies.