... If I backup my 24(+1) Seedphrase from my Ledger,
Bitbox, Trezor or whatever => Am I also backing up my Altcoins?
The only way I imagine this could be possible is, if the wallet software from the
company uses the btc masterkey to generate all other keys on the wallet. Is that
the case and if so: Are there hardware wallets that don't do this?
Generally, a wallet generates all keys for
most coins from the same master key. You should check.