Long TXT record values are a pain to work with, as they need to be quoted in blocks of 255 characters. In terraform, it is only necessary to insert "" separators at those boundaries, but that seems to be too cumbersome to do manually.

So here is a way to just paste the whole record, and let terraform automatically split it up into chunks of max 255 characters:

resource "aws_route53_record" "dkim" {
    name    = "google._domainkey.example.com"
    type    = "TXT"
    zone_id = aws_route53_zone.example_com.zone_id
    ttl     = 300
    records = [
        join("\"\"", regexall(".{1,255}", "v=DKIM1; k=rsa; p=that-super-long-key"))