Thanks for the suggestion. I needed to added the .group parameter because otherwise some functions like sum will perhaps not behave as intended. Interested to hear your thoughts.

summarise_with.R

library(dplyr, warn.conflicts = FALSE)
library(rlang)

summarise_with <- function(.data, ..., .f, .with, .over, .group = TRUE, na.rm = TRUE) {
  
  stopifnot(rlang::is_logical(.group))
  
  if(rlang::quo_is_symbol(rlang::enquo(.with))) {
    .with <- rlang::enquo(.with)
  } else { 
    .with <- rlang::sym(.with) 
  }
  
  if(rlang::quo_is_symbol(rlang::enquo(.over))) {
    .over <- rlang::enquo(.over)
  } else { 
    .over <- rlang::sym(.over) 
  }
  
  if (.group == TRUE) {
    .data %>%
      dplyr::group_by(..., !!.over) %>%
      dplyr::summarise("{{.f}}_{{.with}}" := .f(!!.with, na.rm = na.rm))
  } else {
    .data %>% 
      dplyr::group_by(...) %>% 
      dplyr::summarise("{{.f}}_{{.over}}" := .f(!!.over, !!.with, na.rm = na.rm))
  }
}

add_share_with <- function(.data, ..., .with, na.rm = TRUE) {
  
  if(rlang::quo_is_symbol(rlang::enquo(.with))) {
    .with <- rlang::enquo(.with)
  }  else { 
    .over <- rlang::sym(.with) 
  }
  
  .data %>% 
    dplyr::group_by(...) %>% 
    dplyr::mutate("share_{{.with}}" := !!.with / sum(!!.with, na.rm = na.rm))
}