Hello One Ring Hive mind

So this is the third of my 'Get advice on how to proceed' threads for my ongoing Darkening of Mirkwood campaign. And the last two times have really helped, so thank you in advance!
So last week my group went through the 'Secrets Buried' adventure, in what I think was the most successful session so far. I bulked out the adventure by having them explore the dungeons of Dol Guldur before carrying out the investigations in the High Keep. Their goal in the dungeons below was to find the remains of King Thrain, left behind by Gandalf exactly 100 years prior (Which I thought was a nicely poetic coincidence).
They found the kings skeletal remains, identifiable only by his belt buckle that bore the sigil of the Longbeards. But missing a skull. Now this is a plot thread I've weaved in to the campaign to callback to in a few years. Between the White Council assault and the current year Saruman sent his Dunlending cohort (who shows up in later years in the printed adventure) To seek out thrain, and confirm that the Dwarven ring he bore is not still with his body (Not that that was expected at all) and if not to take his skull to use in conjunction with mystical ways of attempting to divine that ring (I'm not sure whether or not Saruman would have the Palantir at this point?). Anyway, in terms of clues they found a throwing dagger bearing a celticy style Dunlending marking.
Up at the keep they suffered through a lot of corruption tests (To the point of the Dwarf suffering his first bout of Madness, first hacking up the Orc corpse they found in a blinding rage and then striking Radagast when he tried to calm him down, snapping him out of it in a moment of horror) and got through all the clues as written up to the Lake towner body with the woodland realm dossier.
So the players didn't want to move on to the next year straight away, wanting to leap on and investigate as soon as possible. I let them spend a few weeks recovering shadow in Rhosgobel before they struck out to Lake Town to try and uncover what they can.
So far they have found out: The identity of the dead Lake Towner (A merc/dogsbody type character), and at his apartment found a hidden stash of coins as well as a bottle of fine Dorwinion wine (To foreshadow Halbrech the Wine Seller)
And also have found a cartographer that originally made and sold a map they found of Lake town in Dol Guldur, with notes on of it's defenses. He has told them he has only drawn up three maps of laketown over they years and sold them to the Master of Laketown, the Captain of the Guard and a Merchant (Halbrech, but he did not recall the name) The players intend to kick off next session with talking to the Guard captain.
So.. Going forward, I want the players to feel satisfied with their snooping. But Ideally I don't want them to discover Halbrechs identity just yet, the adventure down the line where they stop him from poisoning all the important folk is a cool plot Id like to reach.
So, what kind of trail do you think I should lead them on. And what would you suggest as a satisfying thing to discover through investigation?