ha, it looks like M beat me to the clear straps suggestion! But I think you might be best off with a bra that's marketed as a "convertible" and not a strapless? Because the strapless ones will always be super-tight whether you attach the straps or not. There are bras that are meant to be worn with straps, but what straps you use and how they're attached can change. It'll look like this one.

That might be a good one for everyday for you, too? A lot of people with narrower shoulders like the racerback style, where the straps make an X at the back so they can't slip down.