Under US copyright law, all works are protected by copyright except for US Government works. The concept of "public domain" is not legally well-defined, and is used colloquially to refer to government works, works whose protection has expired, works available to all, and works not copyrightable (such as scientific laws or old software). Under older copyright law, releasing a work without the copyright symbol effectively put the work in the public domain.
One can simply say "I dedicate this work to the public domain", and that is typically taken to be enough. CC0 purports to do this (using more words). However, as far as I can tell, author rights under European law are so strong that it is simply impossible. Releasing works into the public law has to be consistent with other aspects of the law. If you grant an perpetual exclusive right to copy and distribute to a publisher, their right does not go away on your death (a right which they would not have with a real public domain work).
A problem is that an author who makes such a dedication (a bare license) could revoke the license and reassert their copyright. They could be estopped from making that argument. As property, your heirs would inherit the copyright and could (try to) revoke the license. Presumably the courts would not allow them to pursue ostensive infringers. Nevertheless, your plan is neither trivial nor bullet-proof.