I agree with you that the GUI does that for me, but I need to have a dashboard with BTC holding, ETH holding etc... I need to be able to get all the info from somewhere... so I was wondering, if I include one of the many Addresses of my wallet, somehow could I "get also the other addresses associated with the same wallet (read only)...
is it feasible?
With addresses? No... but as mentioned previously... using "Master Public Keys", Yes.
Goto "Wallet -> Information" in your Electrum wallet... and look for the Master Public Key displayed there. Using that you will be able to generate all the addresses from your wallet (including the change addresses).
There will likely be code examples floating around that show how to get from Xpub to address etc.
You can read more about key derivation here:
https://github.com/bitcoin/bips/blob/master/bip-0032.mediawikiAnd you can experiment using Ian Coleman's tool:
https://iancoleman.io/bip39/ (don't recommend that you put your actual seeds or master private/public keys into this, unless you're running a local offline copy etc)