TL;DR Opendkim. Hats off the to folk who do marvelous packing 99.99999% percent of the time.

But sometimes they just get it wrong or at least all documentation regarding the package, does not match up with the behavior the packager intended.
That said, hopefully i will help solve some of these issues here.

I assume you have postfix already working and you are just looking to add dkim support.

Install opendkim

Configure systemd service file

The specific changes here are lines #9 EnvironmentFile #13 -p $SOCKET

Defaults File

Opendkim Config file

Specific changes here are lines #7 hashed out socket file,  it simply does not work.
The only way to get opendkim to honour this setting is passing it to the command line #13 of the service file.

Modify /etc/postfix/main.cf and add/edit the following lines

Create referenced folders for your open dkim keys

TrustedHosts

SingingTable

KeyTable

Create private/public key for signing

Your directory should now look like this

Opendkim Treeview of files

Tell systemd to reload the the daemon files and restart

DNS

Now go modify your DNS, adding a TXT record ‘default._domainkey‘.
Copy and paste everything between the parenthesis. ( everything here ) into the value field of the TXT record

Enjoy

Other resources
https://www.google.com/search?q=opendkim+setup