Please bear in mind that EU postal / parcel systems are not homogenized - run like a single entity. Each country still treats parcel shipments to other EU countries as international shipments. It is a veritable pipe organ.
My best advice is to wait until you arrive in Spain to ship your parcel from the first Correos (Spanish Post Office) you can. The Spanish Correos is inexpensive, reliable and fast. Staying within one national postal system, is definitely easier, and with less drama.
I have had luck over the years sending parcels to hotels along my route of march. Once I make a reservation, I send them an e-mail to ask if they will hold a parcel until my planned arrival. I have never been refused.
Then, from a Spanish post office, send the parcel to you, care of the hotel. Add a note at the bottom that says: "Espere hasta la llegada el 10 de mayo de 2024." (Change the date as appropriate.)
In the past, I have used this method to ship nutritional supplements ad clothing ahead.
You can also use this method to ship things ahead to Casa Ivar at Santiago, to be held until your arrival there. I have also used this method to ship to the hotel I reserved at Santiago. That works too.
Hope this helps.
Tom