The results of the commands you recommended I run are below. Still looks like bad hardware to me.
After iMac was power-cycled and with just the known good Ethernet cable plugged into the Ethernet port, Ethernet port blown out with compressed air, NO USB to Ethernet adapter plugged in:
joe@debian1:~$ date
Thu Sep  4 06:40:38 AM EDT 2025
joe@debian1:~$ sudo ifconfig
[sudo] password for joe: 
enP1p3s15f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::20d:93ff:fe4a:42d2  prefixlen 64  scopeid 0x20<link>
        ether 00:0d:93:4a:42:d2  txqueuelen 1000  (Ethernet)
        RX packets 4  bytes 5564 (5.4 KiB)
        RX errors 3  dropped 7  overruns 0  frame 2
        TX packets 76  bytes 12707 (12.4 KiB)
        TX errors 0  dropped 27 overruns 0  carrier 0  collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 480  bytes 40266 (39.3 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 480  bytes 40266 (39.3 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
joe@debian1:~$ sudo ip link set enP1p3s15f0 up
joe@debian1:~$ sudo dhclient
sudo: dhclient: command not found
joe@debian1:~$ sudo dhcpcd
dhcpcd-10.2.4 starting
dev: loaded udev
DUID 00:01:00:01:30:4c:26:c7:00:0d:93:4a:42:d2
no interfaces have a carrier
joe@debian1:~$ sudo ip link show enP1p3s15f0
2: enP1p3s15f0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
    altname enx000d934a42d2
joe@debian1:~$ sudo dmesg | grep -i eth
[    2.743937] pmac64_cpufreq: Frequency method: SCOM, Voltage method: SMU
[    5.812401] gem 0001:03:0f.0 eth0: Sun GEM (PCI) 10/100/1000BaseT Ethernet 00:0d:93:4a:42:d2
[    8.581065] gem 0001:03:0f.0 enP1p3s15f0: renamed from eth0
[    9.550494] windfarm: SMU failed new fan command falling back to old method
joe@debian1:~$ sudo dmesg | grep enP1p3s15f0
[    8.581065] gem 0001:03:0f.0 enP1p3s15f0: renamed from eth0
[   49.467366] gem 0001:03:0f.0 enP1p3s15f0: Found BCM5221 PHY
[   65.284839] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[   65.284975] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[   66.500717] gem 0001:03:0f.0 enP1p3s15f0: Link down
[   82.308774] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[   82.308936] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[   90.820762] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  107.844850] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  107.844990] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  120.004718] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  139.460825] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  139.460964] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  141.892777] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  144.324829] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  144.324984] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  145.540737] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  147.972807] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  147.972944] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  150.404784] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  154.052775] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  154.052926] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  177.156744] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  192.964824] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  192.964960] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  212.420756] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  228.228763] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  228.228924] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  239.172755] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  254.980784] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  254.980946] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  267.140748] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  282.948784] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  282.948922] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  297.540760] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  315.780854] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  315.780996] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  316.996742] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  320.644820] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  320.644957] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
[  323.076717] gem 0001:03:0f.0 enP1p3s15f0: Link down
[  338.884802] gem 0001:03:0f.0 enP1p3s15f0: Link is up at 10 Mbps, full-duplex
[  338.884955] gem 0001:03:0f.0 enP1p3s15f0: Pause is disabled
joe@debian1:~$ sudo ifconfig
enP1p3s15f0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet6 fe80::20d:93ff:fe4a:42d2  prefixlen 64  scopeid 0x20<link>
        ether 00:0d:93:4a:42:d2  txqueuelen 1000  (Ethernet)
        RX packets 13  bytes 74952 (73.1 KiB)
        RX errors 3  dropped 12  overruns 0  frame 2
        TX packets 196  bytes 31612 (30.8 KiB)
        TX errors 0  dropped 62 overruns 0  carrier 0  collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 941  bytes 78619 (76.7 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 941  bytes 78619 (76.7 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
joe@debian1:~$ sudo ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enP1p3s15f0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
    altname enx000d934a42d2
joe@debian1:~$ 
joe@debian1:~$ sudo ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 
127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: enP1p3s15f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 00:0d:93:4a:42:d2 brd ff:ff:ff:ff:ff:ff
    altname enx000d934a42d2
    inet6 fe80::20d:93ff:fe4a:42d2/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever
joe@debian1:~$ sudo nmcli device status
DEVICE       TYPE      STATE                   CONNECTION 
lo           loopback  connected (externally)  lo         
enP1p3s15f0  ethernet  unavailable             --         
joe@debian1:~$ sudo lspci | grep -i ethernet
0001:03:0f.0 Ethernet controller: Apple Inc. Shasta (Sun GEM)
joe@debian1:~$ 
joe@debian1:~$ date
Thu Sep  4 06:44:50 AM EDT 2025
joe@debian1:~$