check carefully what you signed. If you didn’t sign anything saying otherwise, there’s nothing to prevent you from doing it.

If there’s something, you could still work around it (e.g. remove company secrets).

If the resulting product is provable better, then it’s objectively not the same thing you did for your boss.

After checking all of this, your local FSF might give you free legal advice to get going (keep all notes/correspondence secure for later if anything comes up. It proves you tried to act responsibly).