Honestly, just keep your roughness texture as its own separate map. Here’s how I normally do it if vram is really a concern:
Diffuse/Albedo texture in RGB, no alpha
Normal texture in RGB, no alpha, at half or quarter resolution, whatever I can get away with (normal maps take up more vram)
Detail normal in RGB, no alpha, at very low resolution (64x64, 128x128, 256x256)
Roughness, Alpha, and Metallic (or AO if no metallic or alpha map in its place) all in one RGB texture with no alpha channel, one channel each which then gets separated out in the material editor.
Generally, you want to avoid putting things in the alpha channel unless you’re trying to squeeze out extra information in a single texture sample, since the way textures are compressed means alpha takes as much vram as a full second texture anyway. It’s just more difficult on your end at that point to work with your own assets.
Usually though, it’s not worth the effort when just working out what you want a material to look like. It’s faster to work with textures when they’re still their own separate things. You can just load up the roughness map by itself as a third texture and plug it in directly to the roughness channel on the material
And yeah, you can just ignore the 3d preview in the material editor 90% of the time. It’s only good for seeing if everything’s connected up right, it doesn’t represent what things look like at all.