oK. i've read the route. A point about Rhinog Fawr and the descent gully. If you don't like the look of it continue for about 150 yards and there is a second easier gully. Did that a few weeks back.
As stated the direct route from the cairn in the Bwlch Drws is a demanding climb. If you want an easier option turn down the Drws (west) for 150 yards and take the first stile hopping the stream to get to it. Continue onn towards Llyn Hywel and you will see the gully on your left. It's a grass ascent much easier than the direct route. I found it years ago and only recently did I find that it was a route in Walter Poucher's book of Welsh walks.
I haven't got a photo of the actual gully. First pic is looking back down Cwm Hosan from close to Llyn Hywel. The gully will be visible to your right if you are standing where the pic was taken.
Second pic is of Rhinog Fach from Llyn Hywel. At the extreme left you can see that there is a 'break' in the summit line which the gully leads to.
Apart from saving your legs for the ascent of Y Lllethr, the Cwm Hosan route is more interesting and gives views of the mountain which you may not otherwise see. It probably adds 800 yards to the itinerary.
However you do it you will have a great day.