I have used laticrete's two part product called Spectralock. It is bomb-proof. ( goes into guest bathroom and pokes with screwdriver ) yup, bomb-proof. This was 10 years ago and it has improved. Back then it was offered in maybe 15 colors. Now there are 40.
Make sure your contractor has a good reputation and see examples of his work, because you are NOT fixing that shit. The old grout you could hook out with a special knife. This will require a diamond blade on a grinder

We used another product called Groutboost in the master, maybe 4 years ago. I got talked into it because it was "as good as" epoxy and easier to clean up. It was a liquid that was used in lieu of the water in a standard grout.
Not even close. We mixed according to directions and it is no where close to as hard and sealed as the true epoxy.
If I were staying any length of time in that house I would do the epoxy.