Have used the Cwmrhwyddfor site several times over the years. Generally OK, but facilities very basic if that sort of thing worries you. However, our last visit was spoilt by a load of very loud Scousers which were pitched next to us.
I have got a Jirishanca, which is the 35l version of the MM. Very robust and comfortable, I use it several times a week for both cycling and walking. It has lots of straps and attachment points for attaching stuff to the outside. You can remove most of the external faffage if you don't need it. Not sure about it's exact capacity, as I almost never fill it. However, if I strap the tent to the outside, I can get enough gear in it for an overnighter, although it's a very tight fit.
I went with one of my daughters at the beginning of April. We weren't camping, but experienced some of the highest winds I have ever known. One day the wind literally lifted our Land Cruiser off the road twice. On crossing the road from the hotel to the cafe opposite I was picked up by the wind and blown about 20 yards before I touched down. I have been in winds before where it has been impossible to stand, but I have never been blown 20 yards without touching down!
We had a couple of half days where the wind was 50+ mph - difficult to stand, but not totally mental.
You could be lucky and not experience any extreme winds, but you should at least make contingency plans.