Decidi dar uma mudada no projeto e usei as estrelas como "fases" e dificultei o level up de acordo com o level e fase

  int level = 0;
  int maxLevel = 20;
  int maxStars = 5;
  int nextLevel = 0;

SizedBox(
                  height: 50,
                  width: 60,
                  child: ElevatedButton(
                    onPressed: () {
                      setState(() {
                        for (var i = 0; i < maxStars + 1 - nextLevel; i++) {
                          int randomNumber = (Random().nextInt(10));

                          if (level < 5) {
                            if (randomNumber > 2) {
                              level++;
                              break;
                            }
                          } else if (level < 10) {
                            if (randomNumber > 5) {
                              level++;
                              break;
                            }
                          } else if (level < 15) {
                            if (randomNumber > 6) {
                              level++;
                              break;
                            }
                          } else if (level < 20) {
                            if (randomNumber > 8) {
                              level++;
                              break;
                            }
                          } else if (nextLevel < maxStars) {
                            nextLevel++;
                            level = 0;
                            break;
                          }
                        }
                      });
                    },
                    child: Container(
                      padding: const EdgeInsets.only(bottom: 5),
                      child: Column(
                          mainAxisAlignment: MainAxisAlignment.spaceAround,
                          children: const [
                            Icon(Icons.arrow_drop_up),
                            Text(
                              'Lvl Up',
                              style: TextStyle(fontSize: 10),
                            )
                          ]),
                    ),
                  ),
                )

Color setColor(int nextLevel, int level) {
  if (nextLevel == 0) {
    return Colors.blue.shade400;
  } else if (nextLevel == 1) {
    return Colors.green.shade400;
  } else if (nextLevel == 2) {
    return Colors.amber.shade400;
  } else if (nextLevel == 3) {
    return Colors.orange.shade400;
  } else if (nextLevel == 4) {
    return Colors.red.shade400;
  } else if (nextLevel == 5 && level == 20) {
    return Colors.black;
  } else {
    return Colors.deepPurple;
  }
}