You can do a full day of excellent riding on the Llanwrst end alone, it’s waaaaay better than Dyfi off-piste IMO.
There are a few trails beside the Sawbench car park including Granny Smasher, which are very good (bit gnarly). Jizz Lobber above them is more severe, if you want a real challenge.
Ask people there to point you to the Rugby Club descent (not on TF but amazing).
And also head up the top of the main climb and ride the short trails including “wales never fails”, there are more than are shown on TF and you can quickly loop back round from the bottom.
You’ll spot more off piste bits from the main climb, so have a sniff around the ruined wall on that as well for the start of those.