I have done this exact project twice (replacing shower valve and also the plumbing behind it with pex connected to the old copper pipes with sharkbite connectors). The biggest problem I ran into was that in one instance the section of copper pipe I was trying to connect it to was slightly bent, which was preventing me from getting the sharkbite connector on all the way. Just wanted to mention that because it took a while to realize that was the issue, and that the solution was to just take a few more inches off with the pipe cutter. Using the tool to remove them after they are somewhat on a pipe already is also a lot trickier than they make it look in video examples, so it's a good idea to look closely to make sure the pipe is exactly the right shape first (round).

Sharkbites aren't that expensive really given that you should only need two and the shower valve replacement and other equipment you'll need is much more expensive.

The best part of doing it this way overall was that I didn't even have to think about how the decades old rusted mess that was the previous shower valve worked, just had to remove and dispose of it.