To achieve full anonymity it is better to generate your mnemonic phrase manually, without using online or offline tools whatsoever since they can be still compromised.
You can use a live OS with pre-installed Electrum on an air-gapped PC to create your seed too. After the generation is complete you can wipe the disk if you're paranoid about it. Anything can be compromised but that's why verifying files, airgap pc, etc exist.