The best suffix for -land in Classical Latin is unequivocally -ia. In the vast majority of "lands" take this form: Italia, Hispania, Gallia, Hibernia, Germania, Graecia, Persia, Aegyptia, etc.
-landia to the Romans would only have been acceptable if it were a part of the name of the land given by its inhabitants. Should the Romans have met the English, they might have called their country Inglandia. It's fathomable. But nation-states didn't really exist back then. There were people and their lands. The area where those people lived came directly from the demonym. So Italia is the land where the Itali are, and Graecia is where the Graeci are.
The reality is that many of these countries came into being as countries only the Medieval and later periods, and what the Romans would have called them was supplanted by their neighbors, for whom Latin was not native. So Hibernia, the ancient name for Ireland, was neglected in favor of Irelandia, a combination of Eire (Gaelic) with the -landia ending from English. Finlandia seems to have come through the same way but ultimately from the Norse word for the Suomi, Finnr. Some source I saw, without citation unfortunately, says that the -landia suffix was Swedish. In that case, the -landia would likely be kept if the Romans had met Finland via the Vikings, and even then given their tendency for analogy would still call them the Finni and their land Finnia. (Cf. Tacitus' Fenni in Germ. 46. There is no Finlandia in Tacitus.) This is hypothetical, but it's sticking to what they typically did.
Whether this is "good Latin" or not will depend on how much Medieval and non-native impurities one permits in their language. It simply doesn't exist in ancient Latin. Iceland was (likely) Thule. Ireland was Hibernia. Others were simply terra Xorum where the territory was not clearly defined. From a native Latin perspective, it would be a barbarism to replace the native -ia with -landia. Of course, there aren't any around to argue for their language anymore, so modern Europeans have instead took from their own, more familiar language when attempting to write in Latin, thus displacing the original Latin forms.
By the way, -terre/-terra in French and Italian are calques of the English England. They are not examples of ancient Latin.
