Just to add, the Newtown-Crossgates section of the A483, I hope you aren’t traveling with anyone who is remotely susceptible to car sickness. It’s a great road on the bike, but in a car, no thanks.
My two offerings to you would be to head A41 Whitchurch, then A49 Leominster, A438 to Brecon, then A40/477 to Tenby.
Either that or the previously mentioned Ruthin/Dolgellau/Aberystwyth/Cardigan and down to Tenby.
I am also not a fan of the M5/6 combo, even on a bike where traffic is less of an issue.