While they are used in DHCP, DNS is oblivious of MAC addresses.
Your debug log shows you currently do not have enabled Pi-hole's DHCP server (but it has been in the past).
That would be the correct place to associate names to DHCP clients, potentially overwriting the name a client tries to claim for itself - provided that Pi-hole is your DHCP server.
If your router is your DHCP server, that service falls in your router's scope.
A DHCP server may register a hostname (as presented by a MAC client during DHCP lease negotiation) for an IP address with its co-located DNS server, but there is no specification requirement to do so.
While Pi-hole's DHCP server would automatically create the respective DNS records, your router's DHCP server may not.
You may verify by running a reverse lookup for one of your client IPs directly against your router's IP, e.g.
nslookup 192.168.1.21 192.168.1.1
Your router should return the correct name for 192.168.1.21
.
If it does, you can configure Pi-hole to retrieve local names from your router by enabling Pi-hole's Conditional Forwarding.
If it doesn't, you could consider to create the necessary Local DNS records via Pi-hole's UI, which stores its name/IP associations in /etc/pihole/custom.list
(where you could also bulk edit your definitions).
This has to be done only once, but of course it would only be sensible if your router would always assign the same fixed IP address to a client. In order to succeed with that, you may have to disallow clients to randomize a different MAC address any time they connect to your network.
Client Group Management is the wrong panel to use.
It is intended to define specific clients that need special filtering, differing from the default.
In particular, this means that you won't need to define clients that will be subject to Pi-hole's default filtering group.
Your debug log shows you have defined some 50 clients, but only 5 of them are in a different filter group, i.e. the rest of them (using Pi-hole's default group only) is unnecessary and can be removed.
Unrelated to your question, your debug log shows that you have disabled BLOCK_ICLOUD_PR
.
Note that this would mean that devices browsing via Apple's iCloud Private Relay would by-pass your Pi-hole.
Consider setting it back to true and configure the devices to not enable Private Relay for your home network if you want those devices filtered by Pi-hole.