I'd be very tempted to take the T10 to Corwen.
At Corwen either bus or the preserved (main line size) Llangollen Railway to Llangollen, coming back on the other means of transport.
Or the T3 connection to Bala (a through bus from Wrexham via Llangollen). Take the 2 hour break between buses at Bala (the bus is going on to Barmouth via Dolgellau). Bala is an old market town, in a beautiful location with the lake and a miniature steam train (the Bala Lake railway) then on to Dolgellau for the T2 to Caernarfon via Porthmadog.
Alternatively stay on the bus through Bala and take the lunch stop in Dolgellau.
All those buses are covered by the £7 1Bws ticket/daily cap, same as the Sherpa.
If you did just want to stay on the S1 direct then Llanberis is your break point.