You don't need to use a delivery service. It's true you need to set up SPF/DKIM/DMARC for many peers to accept mail from you, but then you can deliver the mail yourself directly.
Yes, this is true, but I still get better delivery through delivery services. For example a lot of email providers send any email coming from a cloud hosting IP directly to spam.
Even with that most big providers (looking at Outlook) still use IP reputation. Some popular email blacklist providers will list entire /24s if they receive enough abuse complaints so you generally need a high quality provider that takes abuse seriously to prevent getting caught up with a bad neighbor or your own IP block.
After using the same Digital Ocean IP for 8 years, I got unlucky and their /24 block got listed on spamhaus (or mailcop, or similar) and major providers (Outlook and Gmail) stopped accepting messages from my server.