- HWW ist not open source and not full privacy: the company (trezor, ledger etc.) can tell the government how many cryptos I own
This is not true at all.
All hardware wallets are not closed source, ledger is, Trezor is open source.
No hardware wallet can know how much coins you have, especially if you use them with Electrum, or with Tor.
- I have to remember the PIN of the HWW + the seed (instead of only the seed for a cold wallet of electrum).
Electrum also have password, and hardware wallets have optional passphrase.
You don't have to remember seed words, doing that would be stupid and risky, you write them on paper or metal.
Am I correct ? Any objection ? What would be the advantages of an HWW versus Cold Storage through electrum ?
No.
Hinestly, you just wrote a bunch of mambo jumbo.