I would bet that the new M/L Giant would fit you great. It's just been made available, so it wasn't around when you were shopping around for your bike. Look at the geometry chart that I posted above.

The M/L has an effective top tube of 57 cm, which is only 1 cm more than your bike.

The seat tube angle is 73 instead of 73.5 on your Cannondale. If you set your seat on the Giant in the same location relative to the bottom bracket as your Cannondale, that shortens the effective top tube on the Giant by .5 cm relative to your bike. So the overall reach difference is only .5 cm, smaller than any common stem length increment.

The head tube length is 16 vs. 15.4 on your bike, so that's a difference within about 5mm spacer (assuming that the fork lengths and bb drops are similar).

The top of the seat tube is at least an inch less than on your Cannondale, but that's why they make road seatposts a lot longer now than when I started road biking.

So, you are kind of right about the hole in Giant's sizing in the PAST. Look at the increments of top tube lengths in the chart that I posted. Without the M/L, the jump would have been from 55.4 to 58.5 cm, but that hole has been filled. (Though I don't see how a 56 cm effective top tube is "2 cm" away from 58.5 cm. Plus it's darn close to the 55.4 on the M unless you need the extra 1.2 cm head tube length on your Cannondale.) The increments are now around 1.5-2 cm except for between the two largest sizes (2.5 cm) where people have greater choice of good stem lengths anyway. With an increment of 2 cm, you won't be more than 1 cm off in top tube length if you are exactly between sizes, so that's just a 1 cm difference in stem length. Do people complain about having to use a 11 cm vs. 12 cm or 12 vs. 13 stem (1 cm difference)? No, those are all accepted lengths, even for the obsessive.