A difficult question and answer I think with many scattered evidence to be checked. But I will try to give an aspect. As, however, the question sets the term of the already "democratically-elected assemblies", the range is happily narrowed somehow. And I hope that it's implied in the question a small time range after the total roman conquest of 146 BCE.

The one case that probably could meet the criteria of this question is the Aetolean league with its cities.

Its democratic nature has been tracked mainly via inscriptions and the historians Livy and Polybius. And the existence of an assembly of the 'koinon of Aitolon' has been tracked ['boule', 'synedrion']. Though there have been noted elections of magistrates [or even smaller bodies], the attributes of the assembly as a democratically elected one, I think it's a conclusion; the most probable one but still conclusion [on these check inter alia Aitolia and the Aitolian League by Peter Funke in Federalism in Greek Antiquity p. 86ff, where references; besides primary sources mentioned there, Larsen's works deserve to be looked, too, I think].

Regarding league's fate I'll give 3 instances narrated by Roman Livy:

i. In 191 BCE, after the defeat of the battle at Thermopylae, the Aetolians tried to surrender to Rome without being enslaved. And the Roman consul replied that they were defeated and they should be treated as such [Liv. 36 28].

ii. In 187 BCE, and after the defeat of Antiochus III, Manlius Vulso [possibly senator at the time, previously consul], pointed out the granted liberty and immunity of the cities in Greece [Liv. 38 48]. However it's unclear the nature of this claimed liberty and immunity.

iii. In 167 BCE a massacre by the Romans is reported in Aetolia. Victims: 550 'leading citizens' [=principes]. These have been interpreted as members of the assembly. [Liv. 45 28]. So most probably the Aetolean assembly was still in existence after the defeat in the Seleucian war.

Then came the Achaean War between Romans and the Achaean league, the battle of Corinth and the greek defeat [146 BCE]; in which Aetoleans didn't participate.

On this the most relevant source I know on the topic is a Pausanias' short account of the 2nd c CE [Paus. 7.16.9].

Speaking about Lucius Mummius Achaicus, the Roman statesman & general who fought against the [Greek] Aechaean league in 146 BCE, he mentions that after the Roman victory Mummius disarmed Greeks and demolished their city-walls, setting taxes in Rome's favor, too.

Mummius also put an end to democracies while instead he established authorities based on economical criteria. He also forbade city-state confederations.

I can't know for sure if this included Aetoleans. And generally for how long.

But regarding the city-states constistuting the Achaean league:

By this Pausanias speaks of existing democracies at the time. However, maybe it's a little unclear which city-states had well documented at the time democracy and specifically assemblies. Polybius, contemporary of the events, is speeking vaguely of democracy among Achaeans within the league [2.38]. However below he's speaking of tyrants that had joined earlier in [2.44]. So though there's possibly a reference for a democratically-working Achaean council, there's no certainty if it was totally democratically elected. Maybe a conversion was meant to be implied. I can't know.