Surely, someone else found this solution... For ME it's an important discovery.

test-circe.scala

import io.circe.generic.auto._
import io.circe.syntax._

case class MoreThan22(
  a1: Int, a2: Int, a3: Int, a4: Int, a5: Int, a6: Int, a7: Int, a8: Int, a9: Int, a10: Int,
  a11: Int, a12: Int, a13: Int, a14: Int, a15: Int, a16: String, a17: Int, a18: Int, a19: Int, a20: Int,
  a21: Int, a22: Int, a23: Int, a24: Int, a25: Int, a26: Int, a27: ComposedObj, a28: Int, a29: Int, a30: Int
)

case class ComposedObj(country: String, city: String)

val bigJson = MoreThan22(
  1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
  "Fernando",17,18,19,20,21,22,23,24,25,26,
  ComposedObj("Italy", "Milan"),28,29,30
)

val r = bigJson.asJson
// r: io.circe.Json = {
//  "a1" : 1,
//  "a2" : 2,
//  "a3" : 3,
//  "a4" : 4,
//  "a5" : 5,
//  "a6" : 6,
//  "a7" : 7,
//  "a8" : 8,
//  "a9" : 9,
//  "a10" : 10,
//  "a11" : 11,
//  "a12" : 12,
//  "a13" : 13,
//  "a14" : 14,
//  "a15" : 15,
//  "a16" : "Fernando",
//  "a17" : 17,
//  "a18" : 18,
//  "a19" : 19,
//  "a20" : 20,
//  "a21" : 21,
//  "a22" : 22,
//  "a23" : 23,
//  "a24" : 24,
//  "a25" : 25,
//  "a26" : 26,
//  "a27" : {
//    "country" : "Italy",
//    "city" : "Milan"
//  },
//  "a28" : 28,
//  "a29" : 29,
//  "a30" : 30
//}

r.as[MoreThan22]
// res3: io.circe.Decoder.Result[MoreThan22] = Right(MoreThan22(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,Fernando,17,18,19,20,21,22,23,24,25,26,ComposedObj(Italy,Milan),28,29,30))