Spiral steel is not generally recommended if your bust is larger than a C cup.
I made myself a formal dress with a separate internal corselette made from Powernet. I'm not at home to double check right now, but I think I used a combination of spiral and flat steel. I definitely would have used flat steels at center back and the side seams, and definitely spiral steels over the bust for comfort. I just can't remember what I used at the side front and side back seams.
The dress was extremely comfortable to wear for a strapless, I didn't have to pull it up once. The corselette went down to high hip and waistline on the dress was at my natural waist. The powernet was definitely a big part of why it was so comfortable! I originally wanted to go full couture and use swiss net but it proved very difficult to find.
I don't really like rigilene for strapless dresses myself, I don't feel it offers enough support - this is just my preference.
Personally, if I was going to the trouble of making a corselette and all, I would use channels and boning rather than sew on rigilene.
Another option could be cable ties - I've used these successfully in toiles that I have made for various things, and in a pair of 18th century stays. Still need to sew channels for these but cheaper than using steel and then it would be safe to wash in the machine.