As @sdayman says you need a relay or you have to expose your IP, the very purpose of SPF is to prove origin so masking it is just a no-no.
Really I find managing email more hassle than its worth and I’m an email geek. I normally recommend using an ESP for this - my own pick is Mailgun. Free for 10,000pm (30,000 if you sign up via https://www.mailgun.com/google). As well as getting SMTP credentials they have a pretty simple API too. The latter is useful in situations where hosts try to control SMTP ports etc (not relevant to you if you’re using a VPS you control but useful for many).
Mailgun, SendGrid, Postmark are great and if you have Office 365 Personal/Home editions, you’d have a Outlook Premium account which allow personalized domains. I’m sure Google must do the same as well.