It's been a long time since I last posted but I'm trying to catch up despite life keeping me very busy.
So far no snippet has referenced Trellheim, but I've always expected that Sharleyan would use the ICN and ICA to finally reap full revenge for her father, even if it was 19 years ago.
The information we have is very limited; according to OAR and BSRA, the small number of Trellheim pirate "kingdom/princedoms" had less than a half dozen galleys each, not worth combining with the KotTL supported fleet.
Geographically speaking we don't have a complete one piece map of Trellheim, but we can expect most of the pirates are concentrated south of 34' north, given the deep winters that Cherayth experiences at that same latitude (though there might be small villages of escaped slaves), and the probable lack of terraforming by the pirates, although procuring slaves for such hard work might be a secondary purpose for their piracy.
We don't know the tropic zone's northern boundary, but quite tropical Tellesberg is less than 17' south, and more pirates may be on the tropical coastal islands than the continent's actual coast, let alone extending that far inland.
Given the preference of Safehold's commerce save Charis's to navigate along the continental coasts, one wonders if the Charisian preference and expertise for blue water routes was created just to avoid the Trellheim pirates.
One then wonders, given the size and scope of the prewar Charisian commerce, if many of the slaves taken were Charisian, and then why weren't joint naval operations against them made by the Royal Chisholm and Charis navies in the decades before Merlin appeared.
All of this suggests the ICN could now easily destroy the now obsolete and pathetic Trellhiem naval forces very quickly, now pinpointed thanks to all those mysterious seijin spies, just using the city class steel cruisers etc.
All the islands of Safehold belong to Charis, and liberating those along Trellheim's coast would provide refuges for all the liberated slaves protected by ICN naval bases, to finish any terraforming and own for themselves.
Initially invading the continent could be a very quick campaign, intended to drive the likely survivors well beyond the terraformed plots and disperse them into insignificance if they can survive without terraformed vegetables etc.
But given human fallibility, its quite possible that formidable inland pirate princedoms might survive.
Though they wouldn't pose a piratical threat anymore, they might prove useful to keeping the ICA in fighting trim, testing new tactics and weapons, the way the raids from the Raven Lord lands did before the war.
Thus Trellheim could serve Charis and Chisholm for decades to come.

Curiously, according to the snippets, Green Valley/Serebor says they're still 'building down' the army 5 years after the war.
Best wishes to all.