Welsh 3000's

[Joester]

Maybe a bit bonkers but possible. Reckon its about 35 miles ?

Planning now to park near Glen Dena and do the Carnedd section first.

Any idea if theres an easyish way back from Foel Fras ? i.e one that avoids going back over any of the peaks

It couldn't be much easier getting from Foel Fras back to Glan Dena, there is less than 300m of ascent (assuming you have dealt with Yr Elen on the way out to Foel Fras).
Get to the broad ridge above the Black Ladders and descend to Cwm Lloer (pathless but unchallenging), then straight down the side of the stream to the A5.

I advise from a mountain walker's perspective.