Although it's generally used with substitutions of the form s/pattern/replacement/, the perl-based rename command can actually accept any valid perl expressions. So for example (borrowing the initialization from pLumo's answer) you could do:

rename -n -- '

  BEGIN{use Time::Piece; our $ts = Time::Piece->new(shift)}

  $_ = our $ts->strftime("Garden%F_%H-%M-%S.wav");

  $ts += 30

' "$(date -d 2021-05-02T04:10:00 +%s)" *.wav

Remove the -n once you are happy with the suggested renamings.

Note that the files will be stamped in the order that * expands in your locale. If you want another order, then IMHO the simplest way is to switch to the Z-shell (zsh) where you can use glob qualifiers to specify the sort order. For example *.wav(n) or *.wav(.non) to sort plain files numerically by name increasing:

% ls *.wav

1.wav  10.wav  11.wav  12.wav  2.wav  3.wav  4.wav  5.wav  6.wav  7.wav  8.wav  9.wav



% rename -n -- '

  BEGIN{use Time::Piece; our $ts = Time::Piece->new(shift)}

  $_ = our $ts->strftime("Garden%F_%H-%M-%S.wav");

  $ts += 30

' "$(date -d 2021-05-02T04:10:00 +%s)" *.wav(.non)

rename(1.wav, Garden2021-05-02_04-10-00.wav)

rename(2.wav, Garden2021-05-02_04-10-30.wav)

rename(3.wav, Garden2021-05-02_04-11-00.wav)

rename(4.wav, Garden2021-05-02_04-11-30.wav)

rename(5.wav, Garden2021-05-02_04-12-00.wav)

rename(6.wav, Garden2021-05-02_04-12-30.wav)

rename(7.wav, Garden2021-05-02_04-13-00.wav)

rename(8.wav, Garden2021-05-02_04-13-30.wav)

rename(9.wav, Garden2021-05-02_04-14-00.wav)

rename(10.wav, Garden2021-05-02_04-14-30.wav)

rename(11.wav, Garden2021-05-02_04-15-00.wav)

rename(12.wav, Garden2021-05-02_04-15-30.wav)

If you can't use zsh, then you can do the same in other shells using an external sort together with xargs:

printf '%s0' *.wav | sort -zn | xargs -0 rename -n -- '

  BEGIN{use Time::Piece; our $ts = Time::Piece->new(shift)}

  $_ = our $ts->strftime("Garden%F_%H-%M-%S.wav");

  $ts += 30

' "$(date -d 2021-05-02T04:10:00 +%s)"