OP, I don't have another idea for you but I am in a similar spot, figured I would add to the conversation. We are nearing the end of ToD, with limited prep time and irregular sessions. Here's what I have been considering, pending a conversation with the group as to what their preferences are.
If the players prefer keeping their characters then I'm probably going to do a high-level sandbox homebrew in some fashion. The suggestion of the Companion modules is a very good idea, as the stories were intended for this style of game. I seem to recall 4E and 5E DMGs mentioning that the adventures become more "international", if not inter-planar.
I don't see how I can avoid converting an older module. I can certainly improv low-level games, but I have never DMed a high level campaign in my 15 years experience, so I'm not confident I can pull it off.
CM1, Test of the Warlords, looks like a solid resource to get players into assuming the role of rulership. I imagine this as the groundwork for a highlevel homebrew sandbox. Some of the followup modules could be used, too.
Planescape looks very suitable for high-level play, but I imagine might take a lot more time (reading the setting, monster conversions, etc). Though I recall seeing a 4E adventure, written for high level characters, intended to introduce them to Sigil. Rather short but meant as a springboard for homebrew.
An "expedition to hell" to finish off Tiamat would be an amazing story to run, and possibly not more work than other options. The current MM has all the monsters ready to go, just requires some story prep. ToD indicates some of this already (Zariel wants Tiamat out of Hell, but Bel wants to stop Zariel), so intrigue and crazy combat in Hell's very colourful locales would be memorable.
Might convert Dead in Thay. This adventure sounds like an easy story to adjust for post-Tiamat circumstances. Eg, Thay already had invasion plans, PCs must interfere, etc.
Another option is converting the adventures from Dungeons of Dread, now handsomely re-printed in hardcover, and just going through them as one-off adventures.
[Apropo the PCs want to retire and play new 1st level characters, then I will run Against the Slave Lords and continue through the GDQ series.]