terraform replace multiple values in arbitrary non-HCL templated string, using map

I am looking to replicate templatefile(), but with a different regex than the usual ${...}. The source here is js code, which already uses${...} syntax for other purposes, messing up the terraform templating.

E.g. my input map may be:

variable "values" {
  type = map
  default = {
    "a" = "Was A"
    "b" = "Hello world"
  }
}

My input is something like:

This is my input with [a] over and [a] over and [b] multiple times [a] and [b],
and I already use ${somevar} and ${anothervar} for something unrelated.

I want to replace every case of [a] with its value in the map, same for [b].

Sort of:

templatefile_alternate("path/to/file", var.values, "[]")

which says, "don't look for ${a} and ${b}, but instead [a] and [b].

I could use replace, but then I end up having to chain one after the other, and getting a loop is difficult:

out = replace(replace(in, "[a]",var.values[a]), "[b]", var.values[b])

If there are more than 1 or 2 items, it is unmanageable.