I learned to fly at Welshpool, so Snowdonia was home territory and I used to know that area well. So a few thoughts...
Routing via the North coast makes perfect sense, or there's also a road and railway line following the valley from Welshpool to Aberystwyth; I took that route home from Caernarfon once when the weather closed in - easy to follow even in poor vis.
Refuelling possible at Caernarfon, Welshpool, possibly Llanbedr, whichever direction you're going in.
Llyn Trawsfynydd is tiny; I can't see anyone thinking it was the sea...but maybe, if the vis is really bad....
Wx can change scarily quickly in those mountains....