You don’t really need a Magic Band these days if you’re not staying on property, as the Fast Pass system is discontinued due to Covid. The parks will be VERY busy the week of Christmas, so lines will be long. Wait times will be best at rope drop (park open) and before close. If you want to get in as much as possible, I would suggest staying rope drop to park close, and then taking a midday break by leaving the park and heading to one of the Magic Kingdom area resorts via monorail for lunch. That way you get better food (Magic Kingdom park food is notoriously blah) a little break from the parks and monorail ride. Restaurants I would suggest for lunch are the Wave at The Contemporary or the Grand Floridian Cafe. If you really don’t want to leave the park, the restaurants I would suggest are either Skipper Canteen, Liberty Tree Tavern or Be Our Guest. There’s also lots of yummy quick service/snacks available. Have fun!
Edited to add: this is assuming parks won’t close due to capacity as often happens during other holiday weeks in years past. I’m just thinking since Disney has only sold a certain amount of tickets this year that everyone who has bought one will be able to get in throughout the day. If parks are still closing due to capacity, definitely don’t leave the park for lunch!