Reporting back: Problem solved!
Using two of the new test nodes, one in transmit and the other in receive, and each using a brand new E28-2G4M27S with just the trace antenna only, and with the default library settings for each, and with the transmit node in a different room with two closed doors in-between (exactly as before, with the initial testing at the start of this thread), I'm now getting the kind of flawless communication that I had expected all along from LoRa on the 2.4Ghz band:
12:23:12 May 18 2022
V1.0
104_LoRa_Receiver_Detailed_Setup Starting
LoRa Device found
SX1280,PACKET_TYPE_LORA,2444999936hz,SF7,BW406250,CR4:5
SX1280,PACKET_TYPE_LORA,Preamble_12,Explicit,PayloadL_255,CRC_ON,IQ_NORMAL,LNAgain_HighSensitivity
Reg 0 1 2 3 4 5 6 7 8 9 A B C D E F
0x900 80 FF 77 41 20 FA BC 13 C1 80 00 00 00 00 00 61
0x910 9C 44 00 00 00 19 00 00 00 19 87 65 43 21 7F FF
0x920 FF FF FF 00 70 37 12 50 D0 80 00 C0 5F D2 8F 0A
0x930 00 C0 00 00 00 24 00 21 28 B0 30 0D 01 51 63 0C
0x940 58 0B 32 0A 16 24 6B 96 00 18 00 00 00 00 00 00
0x950 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x960 00 00 00 00 00 00 00 00 00 00 FF FF FF FF FF FF
0x970 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF 04
0x980 00 0B 18 70 00 00 00 4C 00 F0 64 00 00 00 00 00
0x990 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x9A0 00 08 EC B8 9D 8A E6 66 04 00 00 00 00 00 00 00
0x9B0 00 08 EC B8 9D 8A E6 66 04 00 00 00 00 00 00 00
0x9C0 00 16 00 3F E8 01 FF FF FF FF 5E 4D 25 10 55 55
0x9D0 55 55 55 55 55 55 55 55 55 55 55 55 55 00 00 00
0x9E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x9F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Receiver ready - RXBUFFER_SIZE 32
5s Hello World 1234567890*,CRC,DAAB,RSSI,-54dBm,SNR,10dB,Length,23,Packets,1,Errors,0,IRQreg,8012
9s Hello World 1234567890*,CRC,DAAB,RSSI,-55dBm,SNR,14dB,Length,23,Packets,2,Errors,0,IRQreg,8012
13s Hello World 1234567890*,CRC,DAAB,RSSI,-55dBm,SNR,10dB,Length,23,Packets,3,Errors,0,IRQreg,8012
18s Hello World 1234567890*,CRC,DAAB,RSSI,-58dBm,SNR,13dB,Length,23,Packets,4,Errors,0,IRQreg,8012
22s Hello World 1234567890*,CRC,DAAB,RSSI,-56dBm,SNR,14dB,Length,23,Packets,5,Errors,0,IRQreg,8012
26s Hello World 1234567890*,CRC,DAAB,RSSI,-52dBm,SNR,13dB,Length,23,Packets,6,Errors,0,IRQreg,8012
30s Hello World 1234567890*,CRC,DAAB,RSSI,-53dBm,SNR,9dB,Length,23,Packets,7,Errors,0,IRQreg,8012
34s Hello World 1234567890*,CRC,DAAB,RSSI,-51dBm,SNR,8dB,Length,23,Packets,8,Errors,0,IRQreg,8012
38s Hello World 1234567890*,CRC,DAAB,RSSI,-54dBm,SNR,13dB,Length,23,Packets,9,Errors,0,IRQreg,8012
42s Hello World 1234567890*,CRC,DAAB,RSSI,-52dBm,SNR,14dB,Length,23,Packets,10,Errors,0,IRQreg,8012
46s Hello World 1234567890*,CRC,DAAB,RSSI,-55dBm,SNR,13dB,Length,23,Packets,11,Errors,0,IRQreg,8012
50s Hello World 1234567890*,CRC,DAAB,RSSI,-55dBm,SNR,12dB,Length,23,Packets,12,Errors,0,IRQreg,8012
54s Hello World 1234567890*,CRC,DAAB,RSSI,-56dBm,SNR,9dB,Length,23,Packets,13,Errors,0,IRQreg,8012
58s Hello World 1234567890*,CRC,DAAB,RSSI,-59dBm,SNR,9dB,Length,23,Packets,14,Errors,0,IRQreg,8012
62s Hello World 1234567890*,CRC,DAAB,RSSI,-59dBm,SNR,13dB,Length,23,Packets,15,Errors,0,IRQreg,8012
66s Hello World 1234567890*,CRC,DAAB,RSSI,-61dBm,SNR,13dB,Length,23,Packets,16,Errors,0,IRQreg,8012
70s Hello World 1234567890*,CRC,DAAB,RSSI,-57dBm,SNR,13dB,Length,23,Packets,17,Errors,0,IRQreg,8012
75s Hello World 1234567890*,CRC,DAAB,RSSI,-54dBm,SNR,14dB,Length,23,Packets,18,Errors,0,IRQreg,8012
79s Hello World 1234567890*,CRC,DAAB,RSSI,-55dBm,SNR,14dB,Length,23,Packets,19,Errors,0,IRQreg,8012
83s Hello World 1234567890*,CRC,DAAB,RSSI,-54dBm,SNR,13dB,Length,23,Packets,20,Errors,0,IRQreg,8012
Notice that now both the RSSI is very good and the SNR is very high as compared to when I was having trouble with the earlier setup.
The E28-2G4M27S is vindicated. Case closed!