I am not an REI employee, but wanted to share my thoughts and observations. What you are looking for is going to be a compromise. The more feature-rich, and comfortable-carry a pack is, the more space and weight it will take up in your 55L pack. You don't want to have to choose not carrying a true necessity on the trip, just to make room for a more cushioned hip belt. I use an Osprey Ultralight Stuff Pack which stuffs down to fist-sized in its own pocket. Sea-to-Summit makes a similar pack that REI carries. These are great in that they take up no space in my pack, but these are little more than bags with shoulder straps. No bells, no whistles. They work well for short day-hikes away from camp or a jaunt through town, but aren't what I'd want to use for an all-day, or multi-day hike.
Are you checking your 55L, or using it as carry-on? Can you use the day-pack as your "personal item" instead of packing it in your 55L? Instead of a separate day-pack, can you use the compression straps on the 55L to cinch down the contents and not be so bulky on the 1 or 2 day trips?
Of the items you listed, the Matador 28 appears to match your requirements most closely, but I have no previous experience with it, or the brand.
This sounds like an amazing trip! Seems as though you are doing all the right things in your planning, just make sure that you don't get too bogged down in overthinking the features you want with those that are suitable (and acceptable) for your intended purpose. Whatever you end up selecting, this will be a trip to remember! Enjoy!
one or more gifts or other benefits from the co-op.