How to vertically merge more than one column based a factor column

Please help find a solution that allows one to merge multiple columns respective to a grouping column. In the reproducible example I want to merge columns v2 and v3 within the respective groups (a,b,c) only. Please use flextable.

# Sample data
dat <- data.frame(
  v1 = c(rep("a", 3), rep("b", 3), rep("c", 3)),
  v2 = c("G1", "G1", "G2", "G2", "G3", "G3", "G4", "G5", "G6"),
  v3 = c(1, 1, 2, 2, 2, 5, 5, 5, 5),
  v4 = c(1:9), stringsAsFactors = FALSE)

These previously answered questions are similar but not quite the answer. This question used a dummy variable but this only works with one other column and is not scalable.

dat$v_dummy <- paste0(dat$v1, dat$v2)
ft <- dat %>% 
  flextable() %>%
  theme_box()
ft <- merge_v(ft, j =c("v1", "v_dummy"), target = c(1:2))

Another question merge cells vertically, but looses data (i.e. col v2 cel G2) in merge.

ft <- dat %>% 
  flextable() %>%
  theme_box()
ft <- merge_v(ft, j = 1, target = 1:3)

Here is an example of the desired result

I think this answer the question:

library(flextable)

# Sample data
dat <- data.frame(
  v1 = c(rep("a", 3), rep("b", 3), rep("c", 3)),
  v2 = c("G1", "G1", "G2", "G2", "G3", "G3", "G4", "G5", "G6"),
  v3 = c(1, 1, 2, 2, 2, 5, 5, 5, 5),
  v4 = c(1:9), stringsAsFactors = FALSE
)

ft <- dat |>
  flextable() %>%
  theme_box() |> 
  merge_v(
    j = c("v1")
  ) |> 
  merge_v(
    j = c("v1", "v2"), 
    target = c("v2"), 
    combine = TRUE
  ) |> 
  merge_v(
    j = c("v1", "v2", "v3"), 
    target = c("v3"), 
    combine = TRUE
  ) 
ft