I routinely pre-make breakfast sandwiches for the week and have pretty much perfected what I like at this point.
English muffins a great for reheating, sourdough I find reheats the best. I typically toast them slightly as well to maintain some crunch
What you came for: Eggs. I switch up between fried eggs and scrambled. I like my fried eggs a little undercooked, so they can be messier to eat, but scrambled eggs require a good bit of cheese to hold them together. Fried is pretty self explanatory since it's still one piece and you just stick it on there. For scrambled I'll mix in some cheese while I cook them, then when I put it on the sandwich it gets a slice or 2 of cheese on top to melt and hold it in place. I've tried using cookie cutters or muffin pans to get egg rounds, but getting a size that also matches your sandwich usually requires buying a special sized thing that's used only for that purpose, which I try to avoid. Plus I've found that having the eggs in a disc shape just seems to not hold up as well over the week compared to scrambled or fried. The texture tends more to rubbery for me for some reason.
Bacon or ham is good as well
Instead of getting sausage patties already shaped, I'll opt for the tubes of raw sausage. like with the eggs, pre-shaped patties are always too small and you don't get full coverage, and you definitely want every bit to have a little of everything. So I get the tubes and stick them in the freezer while I cook most everything else, then when it's close to frozen- but not completely solid, just more firm- take it out and with a serrated knife cut into thick rounds. You'll want them thick because you'll need to press them to expand the diameter. They're gonna lose a lot of that size when the fat cooks out so make them bigger than your sandwich when you press them out. Then you can just pan-sear the patties and finish off in the oven if they're not cooked through yet.
I'll then put the sandwiches together. Depending on how well the scrambled eggs are holding together, I pay stick the bottom half of the sandwiches in the oven for a couple minutes just to melt the cheese slice on the eggs and keep them where they're supposed to be.
Once done and they're all put together, make sure they've cooled off completely and wrap each one in foil (definitely spray some non-stick spray on the foil to make it easier to remove after re-heating). Then in the morning you just toss a foil-wrapped sandwich in the over (I use an air-fryer typically but conventional ovens work as well, but will need a lower temp and take longer. But you can have that warming up while you do everything else to get ready for the day).
It takes some time to set it all up, but I usually set aside a whole day on weekends to do my grocery shopping, cooking and prep for the week. And these sandwiches seem to keep pretty well for me. I've been able to make enough for as far out as 10 days and don't notice an appreciable decline in quality.