I want to customize x-axis label in ggplot2.

library(tidyverse)

set.seed(1)
df = data.frame(SUBJID = 1001:1010,
                Level = sample(c("Normal", "Mild", "Moderate", "Severe"), 10, replace = T),
                Value = runif(10, min = 200, max = 500))

df %>%
  ggplot(aes(Level, Value)) +
  geom_boxplot()

For known X label, usually I use this code to change order.

df$Level = factor(df$Level, levels = c("Normal", "Mild", "Moderate", "Severe"))
  
df %>%
  ggplot(aes(Level, Value)) +
  geom_boxplot()

But I added the subject number in X label, so the label is not a known character.

df %>%
  group_by(Level) %>%
  mutate(
    SUBJN = n_distinct(SUBJID),
    Level_N = paste(Level, "(N=", SUBJN, ")")
  ) %>%
  ggplot(aes(Level_N, Value)) +
  geom_boxplot()

The X label is not a fixed character, so I cannot use the code I previously used.

How to customize order of X label in this situation?

I tried fct_reorder, and the plot was not what I wanted.

df %>%
  group_by(Level) %>%
  mutate(
    SUBJN = n_distinct(SUBJID),
    Level_N = paste(Level, "(N=", SUBJN, ")") |> factor() |> fct_reorder(as.numeric(Level))
  ) %>%
  boxplot(Value ~ Level_N)