Is this a recent kitchen installation? Could you ask cabinet maker to adjust the base units across to close the gaps up a bit more? Even if the splashback needs to be re caulked, better than trying to infil with stainless steel angles.

It is always best not to look too closely at construction! I've discovered that every tradie seems to have the same way of solving problems: "do the first thing I can think of". Sometimes it is fine, other times not. Our stone benchtop is being fitted tomorrow and I'm going to *try* not to look underneath!! Sorry, no actual advice...

We have had composite stone bench tops for many years. First was Askilan (not even sure this is around anymore), then caesarstone which we had for over 15 years. It had one tiny chip at the edge of the under mount sink. It had a gloss finish in a white fleck and never showed a mark or stain. Replaced with a new caesarstone when we renovated the kitchen, this one has a Matt finish and shows more marks but us just as easy to clean - I love it.

We got an engineered stone resurfacing in marble for our bench and splashback. Looks amazing. We went with Granite Transformations in Taren Point (Sydney), very happy with their customer service and speed of installation. We changed our sink and that could be easily done with the benchtop overlay. You could get rid of the ugly bullnose and get a straight edge too. They also do stone, which is a bit cheaper, but marble looks so good. Granite Transformations Sydney South