Dungeons of Drakkenheim. It's a sandbox, faction-based campaign located in a city destroyed by an Eldritch meteor. One of the best sandbox campaigns I've read due to the wealth of information and resources it comes with. Puts pretty much all the campaigns that WOTC have put out to shame.
I would try to get them to clarify what they hope to get out of an TTRPG that they couldn't get out of Gloomhaven, since the latter seems like exactly what they actually want (well, besides the one player that does seem to engage with the story). Tactical combat with a light story that doesn't...
My favorite 3PP adventure is Dungeons of Drakkenheim. It's a great setting with dungeons, factions, and tools presented straightforwardly. It's definitely one of the most straight-out-of-the-box usable adventures I've ever run.
Translation:
"The development and reception of Dungeons & Dragons' 5th edition were marked by significant uncertainty. When I was finalizing the edition, there were many doubts about its future. Many of us at the time felt as though we were writing our own farewell letters. Some even humorously...
"The development and reception of Dungeons & Dragons' 5th edition were marked by significant uncertainty. When I was finalizing the edition, there were many doubts about its future. Many of us at the time felt as though we were writing our own farewell letters. Some even humorously suggested...