Distance wise, traversing the Rhydd Ddu path via Bwlch Main, is an interesting and different alternative to the Miners and PYG tracks.
The only difficulties i can see, would be route finding in poor visibility.
Bwlch Main is quite an exposed section of the upper reaches of the Rhydd Ddu path, and is very exposed in strong winds, but its far far safer than Crib Goch.
There are no real navigation issues using the Rhydd Ddu path to the top of Snowdon in darkness, but its a very strange alternative to reach the top of Snowdon, although its an interesting approach, and probably not used by many attempting the 3000s, as it involves a bit more distance on an already very long day out.
When attempting the Welsh 3000s, you need as much energy in reserve for those final Carneddau miles, and adding more mileage onto a already very long and arduous route, just because you have thought of a novel and different route, makes little sense.
Not that i think using the Rhydd Path is a bad idea, its not, but you want to try and make the welsh 3000s as straight forward as possible.
If the weather is suspect, the Miners or PYG tracks are the best routes to get to the top, as their the quickest, and finding that the wind is too strong via Bwlch Main will put pay to any attempt on the 3000s.
It took me two attempts to complete the 3000s in one day, and on the first attempt back in 1989, it took me nearly 17hrs, far too long, simply because i was unsure of the exact route and shortcuts available, and i had carried insufficient food and water.
Any 3000s attempt is going to leave you very tired at the end, so it makes more sense to reach the top of Snowdon feeling as refreshed as possible.
Experimenting on differing approaches to the top of Snowdon can be left for another day, when you know you will not be walking close on 36+miles.
Ive now managed the full distance in just under 9hrs 23min, but that was only after knowing the route like the back of my hand, and being able to attempt it, when the weather was perfect.
Your main objective is to finish this magnificent walk, and the best solution is to get to the top of Snowdon by the quickest route possible.