However, Electrum, by default, will attempt to create a "Native" SegWit (aka P2WPKH, "bc1" bech32 address) wallet... even when you use a BIP39 seed, it defaults to the "native segwit" option on the Derivation Path selection screen.
To generate the same wallet as your Ledger and/or Trezor, you will need to select either Legacy (if your addresses start with a 1) or the "p2sh-segwit (p2wpkh-p2sh)" option if your Ledger addresses start with a "3":
This is what I was looking for. I was selecting the third option, not the second! Thanks so much for a satisfying conclusion