Hi - this initially may sound like a crazy idea! but there is a Decathlon in Burgos (all-sports store - great quality, low prices) ... you can buy a new bike for about 130 Euros, more or less the same as renting, and when you get to Leon you could go to a park and find some teenagers or homeless people and just give them away - win-win. Though if you do go for the 'buy and gift' option do get three separate receipts for the bikes to hand over to whomever receives them, so that they won't be accused of theft. (You could write on them "I have gifted this bike to ...).
Staff at Decathons are very helpful and would set up the bikes how you like them and the bikes are definitely good enough for Camino, especially as you are 'comfort' riders so will not be racing (which walking pilgrims loathe!). You can order them in advance too.
see here -
https://www.decathlon.es/C-10829-bicicletas/T-654824_654827
If you use Chrome browser it will automatically translate pages into English.