Leon-Santiago-Finisterra-Muxia in 18 days works out to around 25 kilometers a day a do-able daily ration of kilcks – provided you do the first 25 kilometers the day you arrive in Leon and don't look around; provided you don't take a rest day along the way; provided you don't enjoy a day in Santiago nor in Fistera nor in Muxia; provided you don't - God Forfend - become ill or suffer from feet issues. You will be cutting things thin, very thin in my opinion.
Another option, Barcelona to SJPP will take you the better part of a day just to get there and while you might make it on to the Orisson albergue, you will need a confirmed reservation well in advance.
I would start in Pamplona and walk as far as you have allowed, enjoying every bit of scenery, art, and architecture, as well as the camaraderie and spirituality which the Camino may offer while disregarding the kilometers, then bus/train back whenever your time ends.
Santiago is perhaps the drive for walking; I find that my underlying motivation and lasting inspiration is the Camino experience itself.
I have offered my opinion concerning what Finisterre has become perhaps too often, walk first to Muxia!