I don't want to have to symlink each module individually if I could avoid it. What should I do?

Modify whatever magic in MakeMaker (risky) or simply copy (or symlink) each module individually.

cat Makefile.PL
my $to_symlink =  '/blah/blah/legacy/lib';
use File::Find;
use ExtUtils::Command qw[ mkdir copy ];
my @dirs;
my @files;
find ( sub { return if $_ =~ /^\.{1,2}$/;
if( -d $File::Find::name ){
  push @dirs, $File::Find::name;
} else {
  push @files, $File::Find::name;
}
return;},$to_symlink);

s/\Q$to_symlink// for @dirs ;
mkpath @dirs;

for ( @files ){
    my $source = $_;
    s/\Q$to_symlink//;
    cp $source, $_;
}

WriteMakefile( ...
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