Bitcoin seed guide by @QnA has been so far the best tool I've used to create Bitcoin seed offline without a HW.
It might be a bit all over the place at first, it has a lot of different tools in a single html file.
But all you can do with it is great.
-Generate last word for seed
-generate seed from own entropy with different methods
-Generate xpub based on seed
-Add passprhase
-Test passprhase
-Verify is HW is actually using given entropy instead of pre-installed seed.
-Verify multisig
-More I haven even figure out xD