DKIM, DMARC, SPF, rDNS/PTR, EHLO... all correct and verified in the email "original source," BY GOOGLE. Still goes to SPAM folder because **** you, apparently. I hate Google.
I can reply yo emails. It's what jaybone said -- I use a 3rd party outbound (SMTP).
I use Thunderbird as my mail client. In there you can define multiple IMAP/pop accounts (inbound) and multiple SMTP (outbound). So when I click reply, the SMTP I have set for that email address is a 3rd party service that sends emails using my domain.
This does have a privacy implication, so whether that's appropriate for you is for you to decide. The 3rd party can read and save a copy of your outbound email inclusive of any content included with it (like the email you're replying to if you include it).