Routing and DHCP – How Does a Device Choose Between Multiple DHCP Pools?

Well, one way to do this would be to exclude the entire pool from assignment. This would leave the configuration of the pool in there and you would just remove the exclusion if you wanted it to assign addresses again.

ip dhcp excluded-address <first address> <last address>

However, I would just archive a copy of the config and remove the pool.

Configuring a DHCP relay on Cisco routers is pretty simple. You use the ip helper-address <DHCP server address> command on the layer-3 interface. The address for the helper address must be reachable from the router on which the helper address is configured, e.g. the Router0 interface toward Router1.

You are putting the helper address on the physical interface for which there are no VLANs. You need to put the helper address on the individual subinterfaces (each subinterface could use a different DHCP server).

Your DHCP server must have a scope for each of the networks for which it is the DHCP server.

By the way, never use a picture for text in your question. Just copy the text and paste it into your question, highlight the text, and use the Pre-formatted Text button: {}.

Edit:

Your Router0 needs scopes for VLANs 15 and 20:

ip dhcp pool vlan15
 network 195.165.85.96 255.255.255.224
 default-router 195.165.85.126

ip dhcp pool vlan20
 network 195.165.85.128 255.255.255.224
 default-router 195.165.85.158

Router1 needs helper addresses on the interfaces:

interface GigabitEthernet0/1.15
 encapsulation dot1Q 15
 ip address 195.165.85.126 255.255.255.224
 ip helper-address 195.165.85.29

interface GigabitEthernet0/1.20
 encapsulation dot1Q 20
 ip address 195.165.85.158 255.255.255.224
 ip helper-address 195.165.85.29

You also need to make sure that Router0 can ping the G0/1.15 and G0/1.20 interfaces in Router1.