Adding more information to an eCDF plot in R

I used the following code to get a eCDF plot:

df %>% group_by(group1, group2) %>%   
summarise(n = length(unique(sessionID))) %>%
ggplot(aes(n)) +    
stat_ecdf(geom = "step") +
scale_x_continuous(n.breaks = 30) +   
theme_classic()

I would like to add quartiles, mean, and median to the plot, somewhat similar to the plot below.

My suggestion is to just calculate the quantiles outside the plot and use an extra data.frame as input to the annotation layers.

library(ggplot2)

# Dummy data
df <- data.frame(n = rpois(100, lambda = 5))

# Quantiles
q <- c(0.2, 0.5, 0.8)

# data.frame for quantiles
qdf <- data.frame(
  q = factor(q),
  x = c(rep(-Inf, length(q)), rep(quantile(df$n, q), 2)),
  y = c(rep(q, 2), rep(-Inf, length(q)))
)

ggplot(df, aes(n)) +
  stat_ecdf() +
  # Line segments to/from the ecdf line
  geom_path(
    data = qdf,
    aes(x = x, y = y, colour = q),
    linetype = "dotted"
  ) +
  # Labels at x
  geom_text(
    data = subset(qdf, is.finite(x) & is.finite(y)),
    aes(x = x, y = 0.5 * y, label = x, colour = q),
    hjust = -1
  ) +
  # Labels at y
  geom_text(
    data = subset(qdf, is.finite(x) & is.finite(y)),
    aes(x - 0.5 * min(x), y = y, label = y, colour = q),
    vjust = -1
  )

^{Created on 2022-10-27 by the reprex package (v2.0.0)}