How could I safely generate custom seed phrases offline, and which tools are good for this operation?
It's offline so it should be relatively safe compared to generating it online as long as the software's entropy sources aren't rigged.
If you do not have the means to review the software's code, stick to the famous well-reviewed wallets.
To be even more secure, pick an OS that has good RNG since the wallet software will be using it.
Most OS has good pRNG but only those "
open-source" that you can audit.
Of course, the hardware as well but can anyone even verify its integrity without personally building each component?
But generally, keeping the wallet on an "
air-gap" machine should be safe enough.
Generating the seed phrase offline, then using the created wallet online will just defeat that question's purpose.
What software supports generating seed phrases with custom passphrases, and in which software wallets such phrases could be imported?
There are a lot of good Bitcoin-exclusive wallets and almost all supports "
BIP39 passphrase".
Unfortunately, for multi-coin wallet, it's hard to give recommendation to any.