Using phpwee, this is the php script I came up with. Tested on php 7.1.x.

<?php
//http://searchturbine.com/php/phpwee
class WP_HTML_Compression {
    protected $compress_css = true;
    protected $compress_js = true;
    protected $info_comment = true;
    protected $remove_comments = true;
 
    protected $html;
    public function __construct($html) {
      if (!empty($html)) {
    $this->parseHTML($html);
    }
    }
    public function __toString() {
    return $this->html;
    }
    protected function bottomComment($raw, $compressed) {
    $raw = strlen($raw);
    $compressed = strlen($compressed);
    $savings = ($raw-$compressed) / $raw * 100;
    $savings = round($savings, 2);
    return '<!-- HTML Minify | https://hackya.com/us/wp-snippets/html-minify/ | minification '.$savings.'% | original '.$raw.' Bytes, minified '.$compressed.' Bytes -->';
       
    }
    protected function minifyHTML($html) {
    $pattern = '/<(?<script>script).*?<\/script\s*>|<(?<style>style).*?<\/style\s*>|<!(?<comment>--).*?-->|<(?<tag>[\/\w.:-]*)(?:".*?"|\'.*?\'|[^\'">]+)*>|(?<text>((<[^!\/\w.:-])?[^<]*)+)|/si';
    preg_match_all($pattern, $html, $matches, PREG_SET_ORDER);
    $overriding = false;
    $raw_tag = false;
    $html = '';
    foreach ($matches as $token) {
    $tag = (isset($token['tag'])) ? strtolower($token['tag']) : null;
    $content = $token[0];
    if (is_null($tag)) {
    if ( !empty($token['script']) ) {
    $strip = $this->compress_js;
    }
    else if ( !empty($token['style']) ) {
    $strip = $this->compress_css;
    }
    else if ($content == '<!--wp-html-compression no compression-->') {
    $overriding = !$overriding;
    continue;
    }
    else if ($this->remove_comments) {
    if (!$overriding && $raw_tag != 'textarea') {
    $content = preg_replace('/<!--(?!\s*(?:\[if [^\]]+]|<!|>))(?:(?!-->).)*-->/s', '', $content);
    }
    }
    }
    else {
    if ($tag == 'pre' || $tag == 'textarea') {
    $raw_tag = $tag;
    }
    else if ($tag == '/pre' || $tag == '/textarea') {
    $raw_tag = false;
    }
    else {
    if ($raw_tag || $overriding) {
    $strip = false;
    }
    else {
    $strip = true;
    $content = preg_replace('/(\s+)(\w++(?<!\baction|\balt|\bcontent|\bsrc)="")/', '$1', $content);
    $content = str_replace(' />', '/>', $content);
    }
    }
    }
    if ($strip) {
    $content = $this->removeWhiteSpace($content);
    }
    $html .= $content;
    }
    return $html;
    }
    public function parseHTML($html) {
    $this->html = $this->minifyHTML($html);
    if ($this->info_comment) {
    $this->html .= "\n" . $this->bottomComment($html, $this->html);
    }
    }
    protected function removeWhiteSpace($str) {
    $str = str_replace("\t", ' ', $str);
    $str = str_replace("\n",  '', $str);
    $str = str_replace("\r",  '', $str);
    while (stristr($str, '  ')) {
    $str = str_replace('  ', ' ', $str);
    }
    return $str;
    }
}
function wp_html_compression_finish($html) {
    return new WP_HTML_Compression($html);
}
function wp_html_compression_start() {
    ob_start('wp_html_compression_finish');
}
add_action('get_header', 'wp_html_compression_start');
?>

Hope this helps someone. I had a hell of a time getting this to work.

Why am I not using a plugin instead?

For multiple reasons. I want to keep my site simple, free of any plugin dependency. By minimizing the number of plugins I use, I make migration process simple & trouble-free as well.