Half strength would be the same as general freedom, who's a high end intermediate (and who also has a variety of energy projection abilities at his disposal, as well as great speed). Thus I figured that a similar high tier character would be able to have at least twice the strength.

Instead of nerfing his speed completely, would it work to limit his acceleration per second so he'd only be able to achieve mach 1 or 2 during a fight?

The damage output of the galdhr beams can never exceed 3,600 tons at he point of emission (his eyes), so while a full arc attack would be nigh-undodgeable, it would have only a small fraction of the impact force per square inch that a focused attack has. And the farther away the targets are, the less energy per square inch it has as well. Up close, as in less than five meters away, it would definitely hurt like hell, but any high-tier should be able to muster a defense that can block it. It's the focused beams they'd have to worry about.

The Galdhr sense just lets him detect powerful sources of dark energy like the portals.