I checked this site
I tried to access many times. 4/10 times it shows this way. Something is certainly wrong.
I see, you are using lots of .htaccess rule. Probably added with iThemes. I would suggest avoid this plugin at all. Your WP wont get hacked unless if you use some vulnerable or nulled stuff. Keep everything timely updated and use less plugins as possible as, your site should be secure. If you need Firewall protection, try web firewall.
You are using some uneccessary plugin such as XML sitemap. That can be covered by Yoast already.
It seems you are using PHP based page caching. Sorry, this is not so optimal approach. Try WP Rocket or something similar that can work at .htaccess level to serve page in plain HTML quickly from the DISK.
If you need more resources, don’t hesistate trying a different host. Sometime, limiting growth due to technology is not worth.
Your stack seems bit outdated. Try upgrading to PHP 7.3 (If plugin and theme supports) that is much efficient.
For this site, $10/mo plan at DO would be sufficient to handle the per day traffic easily.