You could use local talk and another Mac with ethernet or a floppy drive. Mac file sharing is built in with system 7 and later
/? 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