On my straight-line walk across Wales, I walk through part of the Hafren Forest to the source of the River Severn and bivvied (aka; wild camped) under Craig y Eglwys (SN 80311 893220). This route led me to some very difficult ground around the North end of the Nant y Moch reservoir. The Rhiew valley offers a variety of good ways, which may be more natural routes to the coast than the Severn Way. There is a broad option between Cefn Coch and Tregynon because this area is rich in rights of way. some evidence that the cattle drovers thought this as well. So a coastal route seems to focus on Carno, where you will stumble on Glyndwr's Way, which does it's best to lure you into Machynlleth, a delightful town but in route finding terms on serves the purpose of crossing the Dovey Estuary.
If you get lured onto the Glyndwr's Way, described to me by a rabid Welsh Nationalist, retired school teacher and a most entertaining man as 'The Fictitious Wanderings of a Drunken Welsh Monk', which lures it's adherents away from the best and most exciting stretch of footpath in the Dyliffe - Staylittle area, between SN 87463 92626 and SN 83710 93073, where this narrow and acrobatic trail lands you on the network of land management tracks that are worth studying with Google Earth as well as OS to sort out the way for a Sea Fall.
If I were doing this route I would be tempted towards Pystyll y Llyn, the head of a lovely valley that provides a very scenic way to the Dovey Estuary, but not a true Seafall for your purposes. Sad there is no true route through the Borth Marshes, many years ago I was taken into them by a local poacher come handyman at the Golf Club. But you can skirt along the edge of Cors Fochno from Taliesin. Among memories of the area above were of the Sessile Oak rocky savannah type woodland Artist Valley got named from, IMO, worth trying to identify some of this feature to the trail.
Hope these observations may give you some food for thought if not a clear route.