Packing into the alpha channel is generally a bad idea as it is very expensive to use and should be avoided in most cases. I’m not sure if the same compression settings apply to the alpha channel tho so it might maintain higher quality. I appreciate your suggestion tho.
BC7 seems to retain a lot more information but does have an added cost. I think I’m just going to go back to bringing roughness and AO in as separate greyscale images as they maintain excellent detail and are not that expensive.