Data transfer to older iphone from newer iphone?

Hi everyone,

My partner bought a refurbished iPhone 6 (not a 6s) to replace her broken iPhone SE. She wants to transfer all her apps and data, but we can't get it to work: the iPhone SE runs iOS 14 and the iPhone 6 only goes up to 12.5.4.

She would like to at least transfer all her contacts and the whatsapp and signal chats. iTunes backup and restore does not work, even after updating the iPhone 6 as far as it can be updated. iCloud backup: not enough space on iCloud (we would need 64GB; Would be willing to purchase the space if it meant we could do the backup...)

Any ideas? Is this even possible? No problem if it's a complicated solution...

Thanks very much in advance!