Subequation reference including all letters

I don't think it's good, but…

Why not? Because the purpose of the top level label is to refer to the global number and the subequations are there for the reader to see.

documentclass{article}

usepackage{amsmath}
%usepackage{xparse} % not needed with LaTeX 2020-10-01 or later

ExplSyntaxOn
AddToHook{env/subequations/begin}
 {
  cs_set_eq:NN label __eddy_subequations_label:n
 }
AddToHook{env/subequations/end}
 {
  bool_if:NT l__eddy_subequations_label_defer_bool
   {
    __eddy_subequations_label_do:V l__eddy_subequations_label_tl
   }
 }

bool_new:N l__eddy_subequations_label_defer_bool
tl_new:N l__eddy_subequations_label_tl

cs_new_protected:Nn __eddy_subequations_label:n
 {
  use:c { @bsphack }
  bool_set_true:N l__eddy_subequations_label_defer_bool
  tl_set:Nn l__eddy_subequations_label_tl { #1 }
  cs_set_eq:Nc label { ltx@label }
  use:c { @esphack }
 }
cs_new_protected:Nn __eddy_subequations_label_do:n
 {
  int_case:nnF { value{equation} }
   {
    {1}{ __eddy_subequations_label_aux:nn { #1 } { a } } % just in case
    {2}{ __eddy_subequations_label_aux:nn { #1 } { a,b } }
   }
   { __eddy_subequations_label_aux:nn { #1 } { a-int_to_alph:n { value{equation} } } }
 }
cs_generate_variant:Nn __eddy_subequations_label_do:n { V }
cs_new_protected:Nn __eddy_subequations_label_aux:nn
 {
  tl_set:cx { @currentlabel } { theparentequation #2 }
  label{#1}
 }
ExplSyntaxOff

begin{document}

begin{subequations} label{eqn1}
    begin{align}
        a&=b    \
        c&=d
    end{align}
end{subequations}

begin{subequations} label{eqn2}
    begin{align}
        a&=b    \
        c&=d    \
        e&=f
    end{align}
end{subequations}

The reference `eqref{eqn1}' I would like to appear as `(1a,b)' 
and the reference `eqref{eqn2}' I would like to appear as `(2a-c)'.

end{document}

The idea is that the first label command inside subequations is for setting the global reference. However, we need to defer the setting when the full number of subequations is known. So label is redefined to do the job and then it returns to its normal status.

You need this first label if you plan to use label also in environments that are not math displays inside the subequations environments, such as enumerate, for instance.

The deferred job suitably sets @currentlabel and issues label.

Note: this won't set the anchor for hyperref at the beginning of the subequations environment. That would cost much more.