Where are you in Texas?

Fiberglass is not technically out. Marvin does not make a 1/2 round in fiberglass but they do make on in wood.

You can check with other fiberglass manufacturers that may or may not make the half round in fiberglass.

Thermally broken aluminum is a very good option for Texas and has a proven track record. Don Young makes a pretty good aluminum product in Texas.

RBA is a "pretty" window but offers no material performance advantage over fiberglass, vinyl, aluminum, or wood for that matter. The RBA window performance is average, at best, and their warranty is below average when compared to the better brands out there.

Vinyl is still a very good option for your climate if it is good vinyl. The websites that are hosted by Renewal that talk about vinyl chipping, warping, fading, etc are pure bunk.

Regardless of material, you can find good and bad in each material. Take a quick survey of the board and you will find people with horror stories about wood, vinyl, fiberglass, aluminum, RBA, etc.

Fibrex by RBA is extremely expensive, but had an impressive sales pitch.

I wouldn't judge Andersen based on an RBA sales pitch. Find a regular Andersen dealer for pricing and installer recommendations if you liked the Fibrex.

Try looking at Duxton and InLine Fibreglass, they make both 1/2 rounds and circular windows out of fiberglass. They may be pricey...