Backuptrans is the best solution I’ve found for this, done 3 successful migrations with it.

You’ll need to also use https://github.com/YuvrajRaghuvanshiS/WhatsApp-Key-Database-... which makes the process easier than it used to be - no need for an android emulator or second rooted phone

What you’re paying for with Backuptrans is mostly the conversion between database formats, I’m not aware of an open source solution to that. The extracting/decrypting/viewing of android messages and editing iPhone backups can all be done with other tools