Moving to Spain 'with your possessions' from outside the EU customs and tax territory (UK Post Brexit)
Very helpful member
Finland have kindly supplied this excellent guide & links to all the documents you require moving to any EU country, 'which includes Spain, it explains where exemptions apply. (Beware without the correct documentation your belongings could be subject to import duty normally the local VAT rate on the value)
https://tulli.fi/en/private-persons/moving/moving-to-finland-from-a-non-eu-country