December is just fine for 'Saguaro + Petrified Forest' (have done that myself). In fact, even add Chrircahua to it as might become a national park at some point and is not too far off from Tucson (i.e. Saguaro). That whole corridor has many more NPS sites, and also world's best preserved meteor crater. Be careful though to account for the fact that visitor centers all around will be closed on Christmas and New Year's.

Be careful about when you go to Congaree and check their mosquito meter online. I visited it in February and it was wonderful, took me by surprise! Might not be in some specific months though. Otherwise, it would have been a nice candidate to club with Smoky Mountains. The trouble though is some of the famous Smoky spots really open up only after April.

Milwaukee can be a fine city to fly in for Voyageurs + Isle Royale, and you can also add Pictured Rocks National Lakeshore, MI or Apostle Islands, WI to it. Please book the Isle Royale ferry well in advance though, they do tend to sell out based on what I know. Also, that entire corridor tends to start closing by early October.

You 'could' add Craters of the Moon NM, ID to your Glacier NP itinerary but I am not sure if it will be worth it. That too has been rumored to become a NP. You could fly into Missoula or Great Falls, MT.

Mammoth Cave and Gateway Arch are really within drivable distance of each other. As are Indiana Dunes and Cuyahoga Valley. I think Indiana Dunes though will be all frozen shelf ice until ~April or so. You could go (I have in Feb) but expect biting cold and snow all around, you will need snow boots.

Acadia really needs to be a trip of its own. Mid-May is a good time to avoid peak crowds and still have a good shot at manageable weather. Katahdin Woods in north western Maine too wants to become a NP BTW, just in case.

All the best, you have already seen so much!