* Run https://documentation.cpanel.net/display/ALD/Installation+Guide+-+cPanel+DNSONLY+Installation

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You can also use VestaCP dns cluster.

5 minutes of actual work, and a 30 minutes while cPanel dnsonly installs

It depends on how well you know these things. It takes 15 minutes for someone who knows how the whole thing works (bind9 + a bunch of zone files). Yeah, two hours for cPanel DNS ONLY.

Let's say you have two vps and you have two servers shared you want to set up dns for them and configure them how long will that take 1 or 2 hours?

You can geolocate them around the world so if ns1 is down in say the US then ns2 in the EU handles DNS requests.

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Because I couldn't make slave dns work, I currently have only one dns server on my personal/friends/family server and both ns's are redirected to that ip.

If one or even both of your DNS servers are down for an hour or so it's no big deal, but think of the impact if your web server was offline for this long!

Thanks so you mean two vps should be set up with two different cpanel dns

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You mean ns2 is set up in another cpanel dns completely different server?

What if the server that has cpanel dns go down? where should the server be located and are there better paid alternatives?

All the nodes will update the DNS Cluster; e.g.

They install cPanel DNS Only on ns1 and ns2 and setup DNS Clusters to do that.

* Reverse DNS

4 IPv4 on Private VLAN with Reverse DNS

"5.1 - "rec_new" -- Add a DNS record" on https://www.cloudflare.com/docs/client-api.html

@Aene I've done this, and I did what @vfuse said, create a wildcard DNS record to point to a server (or a cluster of servers/load balancers, if needed), then do the actual work on the PHP/server sides