Welcome
First up, the depth is amazing! You've got the largest of the super-faint outer chevrons, and the still-fainter ones are starting to show. Excellent bordering on remarkable.
Mapping H-alpha to red and OIII to blue-green is legit and solid.
Using H-alpha as a luminance layer is fraught with traps. The most obvious is any area that is a pure lovely brilliant OIII with no H-alpha will come out black.
You mentioned that the SII was scanty, and that makes it hard (but not impossible, given the great depth you've achieved) to do an SHO rendition without ending up with bloated red rings round the stars from the SII.
One very labour intensive way around (routine for us for all narrowband images) is to separate the image into stars and nebulosity. You can then process the stars and the nebulosity (which is the important thing!) completely separately, getting the colour balance right in the nebulosity, applying sharpening and then reassembling (just adding the two together again).
Much quicker and easier (but not our favourite approach): I'd suggest that you either scrap the SII, or perhaps map it to its natural red if you don't want to throw it out, and you'll have a nice almost-natural-colour narrowband image.
Very best,Mike
