You could try the following. Please ensure you have done a FULL backup, and you are ready to revert if needed.
Add the following to your conf_global.php in the
'sql_utf8mb4' => true,
example
<?php
$INFO = array (
'sql_host' => 'localhost',
'sql_database' => 'stuff',
'sql_user' => 'stuff',
'sql_port' => NULL,
'sql_socket' => NULL,
'sql_tbl_prefix' => NULL,
'sql_utf8mb4' => true,
'board_start' => 1677243260,
'installed' => true,
'base_url' => 'http://location',
'guest_group' => 2,
'member_group' => 3,
'admin_group' => 4,
);
Then try to do the upgrade to the latest release, without doing the UTF8mb4 update