On breaking things

Implementation defined and undefined behavior exists for a good reason, without them C could not be as versatile or performant as it is across multiple platforms. Sure it can be a pain to deal with, but this is trade-off which lies at the core of C's design philosophy.

If you don't want to deal with ensuring that memory regions don't overlap then you are certainly welcome to use memmove. Although even then, if you are inadvertently copying between overlapping regions there's a chance you'll still have a bug because you really didn't want to clobber the source region.

Frankly, it sounds like what you want isn't C at all. There are plenty of high level languages which will (attempt to) give you safe and consistent behavior across all supported platforms. Of course, to do this those languages accept a different set of trade-offs from C, but if you really don't want to have to deal with implementation defined or undefined behavior I expect you will find them a better match for your needs.