I’m off on jolly bobs in a few weeks to Criccieth, by road it’s 23miles to CYB.

Does anyone on here know of any off-road trails / paths that’ll get me there so the wife can drive over in the afternoon and meet me for lunch

Maybe not off-road, but take a look at the sustrans map. I think you should be able to link the two on some scenic back road or cycle paths.

That would do, just so long as I can avoid main roads

Not much between cricieth and Porthmadog but once in port go on to to the cycle track and on to penryndaedraeth .
Cross the toll bridge turn left towards meantwrog , when you see a small hydro power station turn right up a steep Tarmac climb with gates up to gellilydan. This will save going on the main a470.
From here you can ride to cyb or if more adventurous head up a steep climb up to tomen y mur the old roman amphitheatre and pick up a bridle path through a remote area to the back end of trawsfynydd.
You can then ride down the a470 and if you still have energy ride the trails in cyb.

Nice one, thanks