Every 2 minutes 0.debian.pool.ntp.org queries by both Raspberry Pi's in my network

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For Pi-OS/Raspbian, the default network manager is called dhcpcd and is responsible for what ends up in resolv.conf.
If the host is configured to acquire IP details automatically via DHCP, dhcpcd will configure the nameserver(s) in resolv.conf that are advertised via DHCP.
You can see/check those if run below (10.0.0.2 & 10.0.0.4 are both running Pi-hole):

pi@ph5b:~ $ pihole-FTL dhcp-discover
Scanning all your interfaces for DHCP servers
[..]
   dns-server: 10.0.0.2
   dns-server: 10.0.0.4

If instead have configured static IP details on the host like below:

pi@ph5b:~ $ tail /etc/dhcpcd.conf
[..]
interface eth0
  static ip_address=10.0.0.4/24
  static routers=10.0.0.1
  static domain_name=home.dehakkelaar.nl
  static domain_name_servers=10.0.0.1

The domain_name_servers= values will end up in resolv.conf.

In addition to that, if IPv6 is supported and enabled on your router for your LAN , clients normally also receive DNS server(s) via IPv6 router advertisement(RA).
So make sure your router is advertising Pi-hole's IPv6 ULA address for DNS!

Am not that familiar with diagnosing IPv6 RA DNS because I dont have IPv6 enabled on my LAN.
But you could check on clients what DNS server IP's are configured.
Below some hints: