2/ TXT record in the domain (used by SPF). This is (arguably) the
simplest but collides badly with other uses of TXT.
I disagree that, in the case of SPF, this collides "badly" with other
uses of TXT.
But the collisions reduce performance both for SPF lookups and for any
colliding lookups.
The DNS group at one time had one KEY record for all DNS keys for all
applications. They switched to make it DNSSEC specific for this exact
reason.
The SPF group on its own could probably have got _spf deployed if they
had insisted. The IETF can definitely get _marid.$domain deployed.
Phill