I’ve hosted mine for decades. Reputation has nothing to do with spam filtering, but you do need at the very least SPF records and eventually* you’ll need full DMARC. the issue is that, without DMARC, any server on the internet can claim to be a valid mail server for your domain. DMARC lets you restrict it to your own servers.

Self hosting email isn’t hard; properly securing and protecting it is more work. I suggest looking into an all-in-one solution like Maddy, which provides IMAP and SMTP, and which make the server-side effort of DMARC easier. You can cobble together everything, too; it’s not hard, but there are more moving parts, more configuration files and file formats to learn, and more pitfalls in setting it up and getting it to all work together correctly.

Like, soon. the longer you wait, the more likely some waste of space on the internet will spoof your domain. Get it set up and working first, then do DMARC the next day. Or do it all in one go, it’s just a bigger bite to take all at once, and it isn’t strictly necessary: you can do it in steps, as long as you don’t delay DMARC by too much.

FIFY