Improve Experience upon "503 Service Temporarily Unavailable"

First of all, why do we not use replica upgrade strategies for zero-downtime upgrade?

Custom 503

Either way, it would be great to have a custom 503 page. I regularly get users complaining about a service being "down" even though the page clearly says "Temporarily Unavailable".

How about a more friendly maintenance page with some nice illustration and text, like:

We are upgrading this service to provide you the best experience :)

If this message persists for more than a few minutes, contact your administrator at ADMIN_EMAIL

I can also recommend to take some inspiration from HestiaCP, they have some quite nice placeholders: