Hi, Quinranda,
I've walked this route several times and always enjoy it. Whether to push it or not will of course depend on you, but I last walked this camino in 2014 at the age of 64 and was comfortable walking in four days to Muxia and Finisterre. I think the key is to push the first day beyond Negreira. It's a nice little town, has some good restaurants and good accommodation, but it's only 21 km. Adding 12 more on to get to Vilaserio, where there is a nice private albergue (and also a very basic public albergue) makes the rest of the stages easier. I have gone first to Muxia and first to Finisterre, and though I like Muxia better than Finisterre, it does feel more "right" to end in Finisterre. So that means that my Day 2 from Vilaserio is the great albergue in Dumbria. People in town are fabulous, the albergue is awesome. Day 3 into Muxia, and Day 4 out to Finisterre.
Now there is a downside to walking first to Muxia, and that is that unless you walk back from Finisterre to Santiago, you will not get the beautiful coastal views and walk that you get when you walk from Santiago to Finisterre. That's the side of the triangle that the "Muxia first" option doesn't include.
In 2012, I walked first to Muxia and Finisterre and then went into Santiago for the July 24 festivities. I didn't find the crowds to be much larger than on earlier caminos. I probably called Vilaserio when I got to Negreira, just to make sure they had rooms, but didn't make any other reservations. My blog has some details on that
http://caminoolvidado.blogspot.com/
I think you will be happy you added this on to the end of your Camino. Buen camino, Laurie