(Note my two biases from the outset: 1) I am not as enamoured with the coast in general as most people seem to be unless it's spectacular, like the Rota Vicentina; and 2) I really loved the Variante Espiritual.)
I have walked both. The part you have already done is the better part; your problem here is that your first camino was too good!
Starting from Baiona is only one more stage before Vigo so that doesn't give you much new walking. A Guarda gives you two short stages (or one long one) more than that.
Yes, there is some road walking, and it's a main road (PO-552) but there is a 'pilgrim lane' (for lack of a better term) at all times - a different coloured shoulder lane. There are views of the sea but not all coastline is created equal and personally I didn't think this stretch was that impressive. It might depend on how drawn you are to the ocean. Between A Guarda and Baiona, most of it looks like this, so you can make of that what you will:
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That said, there are some nice highlights. The Oia monastery is one of those (although the visit is by guided tour in Spanish only), and La Cala Inn in Oia is a wonderful place to stay. If you're into sea glass (or think you might be!), the 'beach of the crystals' between Oia and Baiona is a detour from the camino but worth going to. Baiona itself has a nice castle and a long waterfront that includes a couple of small city beaches and a marina. Between Baiona and Vigo (if you take the coastal option) the coastline is less rocky and contains quite a few low-key beaches.
My suggestion for a similar length extension would be to start in Ponte de Lima on the central, from where it's 3-4 stages to Redondela, which you would have been through. This gives you Ponte de Lima itself, a very nice town, plus Valença and Tui on either side of the border and the bridge crossing between them - both are also very interesting. You will also get to walk through a lot of vineyards on this stretch, something I always enjoy.