Yes you are right ELECTRUM actually doesn't generate legacy addresses by default. If during the process of setting up your wallet you create a standard Electrum wallet what you will get are segwit addressses both as your receiving addresses and your change addresses. And like you said , it's like that obviously for efficiency of transactions so as to make sure to save fees and also block space.
Anyways back then when I was still making use of Mycelium as my regular hot wallet, Mycelium actually creates HD wallets which I believe are BIP39 wallets . However you have the function to be able to alternate between any address you wish to make use of ranging from legacy address, to SegWit supported address and SEGWIT native addresses. I think importing a BIP39 seed to Electrum should do same. I've imported BIP39 seed to Electrum before though but I didn't take note if they displayed too . I'll try it out during my free time though.
Yes, Mycelium is a wallet that follows the established BIP39 standard.
While Mycelium allows you to switch between the address types and derivation paths: "BIP44, BIP49 and BIP84", the same doesn't happen with Electrum.
Electrum allows you to restore wallets in the aforementioned derivation paths and address types, but you cannot freely switch between them. To access the same wallet (seed) with a different derivation path, you must do the restoration process again and choose the desired address type: legacy, nested-segwit and native segwit, generating a new keystore.
The same goes for other wallets such as Sparrow, with the difference that Sparrow supports taproot for receiving in addition to sending.