Hi Mike, we are in a similar situation to you & we have sold our UK property.
We arrived at the Portsmouth yesterday to board the ferry to Santander. We have everything you mentioned, we also had copies of our applications of Residencia plus a letter from our solicitor saying our property is sold. I also asked our Spanish solicitor for a letter but he didnt come good. Anyway they were happy with all our paperwork but they didnt let us board because.......we were travelling with our 2 pet cats & the vet missed stamping & signing two pages of the new pet certificate But they kindly rebooked us for next weeks crossing & made a note about the cats certificate.
I had to phone Brittany last week to add a cat on as you can only book one at a time on their website. The lady said to me did we have our green cards & I said yes but like you we need to get there to complete it all.
We originally wanted to travel through France but I heard they weren't accepting PCR covid tests they want you to have a more in depth one, very expensive plus it also says on the web you had to isolate for 7 days on arrival which isn't ideal for us with the cats.
I say go for it via Santander & try to get a Solicitor's letter confirming your appointment dates. I'm sure you will be fine.
If you have any questions or want to chat just message me. Good luck
Sue