Falsecolor raster from multiple rasters in QGIS

How can I combine multiple single band rasters into a falsecolor RGB background in QGIS?

I can, of course, use the multiband color renderer to select specific bands, call them A1,A2,A3 from a single raster file source A to R,G,B channels. But I have the desired bands in different files, so it’s R=A1, G=B2, B=C1 that I need.

For the moment, I am creating and maintaining a monster .vrt with A1…An,B1…Bn,C1…Cn munged into one, and then using the multiband color renderer. This requires annoying editing of an XML file to add <Description> tags and to overcome to inability to add different -b switches with -separate in the gdalbuiltvrt call the QGIS Build virtual raster... algorithm uses. I’m hoping for a way where I could assign the right rendering settings to each layer in the QGIS layer tree so that they would individually render as in the 3 rightmost images above, and when all 3 are checkmarked in the layer tree they would add as at left.

I’ve tried using the Singleband grey renderer for each input layer and colorizing it in Layer rendering settings. But that keeps whites white and only shifts the midgreys. So I created my own color maps from #00000 to #ff0000 (etc.). But now the topmost layer wins, unless I adjust transparency in which case the color gets washed out. So I’ve been playing with the blending mode (e.g. Darken and Screen as opposed to Normal) but can’t seem to get the logic right.

This ought to be simple, so I’m probably barking up the wrong tree!

BTW, the reason for this is exploration of different spectral bands and ratios across a time sequence of Sentinel 2 images to discover what combination of data will best pinpoint forest insect damage.