https://apple.fandom.com/wiki/Serial_port links to "Macintosh Serial Throughput" (1998)
https://lowendmac.com/1998/macintosh-serial-throughput/ which mentions a Farallon Mac serial to Ethernet adapter.
/? farallon Mac serial port Ethernet
https://www.google.com/search?q=farallon+mac+serial+port+eth... : $50 and no FreePPP or Zmodem or AppleScript serial port to a [soldered] cable to an Arduino/RP2040W/ESP32 (which is probably faster than the old Mac).
https://unix.stackexchange.com/a/548343/44804 mentions minicom and gnu screen, and the need to implement e.g. file-level checksums because over serial like ROM flashing