Peter you are not far off with your plans i’d say….
The red/black at llandegla are fine,fast, swoopy and fun.
At Coed y Brenin I would do the Fox trail first then have some grub then go and do the MBR which gives good bang for buck and means you get to ride some sections from the Fox again which will allow you to test yourself on sections you have ridden before…
It is a fair drive from there down to Afan… unless you are from ‘down south’ you would be better of exploring Penmachno but as you say you have accomadation booked etc then go to Afan and do the Wall, then drive up the road to gyncorrig and get some lunch and if you feel ok do Whites.
IMHO you are better doing shorter rides with energy to put into the good sections than banging in miles and height gain only to be knackered and not enjoy the good sections or worse still have a nasty off due to being knackered….
Either way have a great time.
For the record I love The Beast and Penmachno is one of my favourite places not just in the UK but anywhere….. in fact Wales is.