Core is at least as reliable as anything else on SmartThings, and probably a little more so. So Core isn’t likely to be the problem. SmartThings itself, on the other hand, does have some reliability issues, so if this is an absolutely critical use case for you, it’s probably not a good match to SmartThings to begin with.
That said, if you feel that SmartThings would meet your requirements, you can do this without Core by daisychaining a virtual switch but at this point your best option is going to be core or webcore because they will let you set up additional conditions like time of day much more easily.
