I know the roads - and the timings are about right. It is quite a packed day & as Parva Farmouse is only a few minutes drive from the ruins of Tintern Abbey, you perhaps could see Tintern the following morning - when the sun will be in a different place. Having said that, I have never paid to go into Tintern Abbey as it is best seen from outside - and does not take long to view - but, it is impressive.
You leave the M4 at Junction 33 for St.Fagans and take the A4232 expressway. Not long after you pass under electricity pylons, you will see the brown signs for St.Fagans - SLOW DOWN in the slip lane as the turning is very sharp (10mph advised). St.Fagans is undergoing a big new project but the Castle & grounds are as good as ever.
Now, when you leave St.Fagans for Cardiff Castle, you need to exit via the left fork from the car park and turn left at first junction to go up the hill - bearing right at top of hill. (DO NOT cross the level crossing - even if the satnav tells you to).Then just carry straight on through every junction for about 3 miles. Eventually you see parkland on your left & go down Cathedral Road - which has large Victorian Houses. Look out for signs saying 'Sophia Gardens'. (It will be the first left after 'The Cricketers' pub). At mini roundabout turn right to enter car park. (You must decide how long you are going to stay & pay in advance at a machine & put ticket in windscreen - I would allow 3 hours). Then, it is a 10 minute walk over the footbridge across the river & through a park exiting by the Pettigrew Tea Rooms.
An alternative to the above route is to leave St.Fagans the same way which which you entered and turn left when you reach the A4232 expressway. It is then a 13 minute drive to Cardiff Bay.http://www.visitwales.com/explore/south-wales/cardiff/must-do-cardiff-bay
So, you could see this area before driving up to the city center & Cardiff Castle. (Coming up from the Bay, the car park under John Lewis is probably your best bet - with a 10 minute walk through modern shopping malls and /or/ Victorian Arcades to reach Cardiff Castle). Ferries also operate from Mermaid Quay almost to the Castle but these take 30 minutes each way.
On the way to Caerphilly, you may wish to visit Castell Coch - which is just N of M4 J32.http://cadw.wales.gov.uk/daysout/castell-coch/?lang=en
A short cut from Castell Coch to Caerphilly takes some narrow back roads. If you are confident to use these - turn hard left when coming down the drive from Castell Coch. At the 'Black Cock' Inn - bear right. Turn left at next junction for Caerphilly. The town is not up to much but the castle is a massive medieval fortress surrounded by a moat.
Try and find the routes on Google Earth.