I would try sharkbite quick-connect fittings. Don’t listen to naysayers, they are permanent when used properly. Use a mini tube cutter on the copper pipe. The trick to using sharkbite is to line them up straight, and forcefully jammimg them on.
Every part you could conceivably need is available. You can put them on copper on one end, and transition to pex pipe out the other. Hope you try it.
Sharkbites are not a bad idea. Make sure if you are using 1/2” OD copper that you use a smaller tubing cutter with a sharp wheel. MAKE SURE you deburr the inside of the pipe or you will have some issues with corrosion. The harder the water the worse the corrosion will be.
Home Depot & other big box stores sell a nice “Compression sleeve puller” and you will thank yourself for making that purchase.
https://www.youtube.com/watch?v=WEM_Ga9O5xA
For the outside you can use emery cloth or even easier is an outer pipe cleaner brush....they are a bit less aggressive and create a better seal for a SharkBite than emery cloth.