My main goal is to generate BTC address on my own with javascript on a browser only without being connected tto the internet?
Do not use a browser to generate the seed phrase. Use text editor on an airgapped device.
About Electrum : Do you know if I can use it in tail OS and if it can be used like Ian Coleman to generate keys and mnemonic phrase only.
These are the OS that you can use with Electrum:
https://electrum.org/#download