Behind the scenes, John Cardinal of GedSite and I have been discussing various issues associated with this scenario.
One was a technical problem with custom Family Attributes and another was the options for those partnerships.
A few concepts need to be clarified.
Much of the way a family couple is represented in FH Diagrams and Reports is due to the Family record that establishes the partnership between the couple regardless of whether there are any Family facts at all. To see that, experiment with a new Family couple without any Family facts, i.e. no Marriage standard event, no Partners custom event, etc.
The way GedSite represents such factless family couples may be different and should be part of the experiment.
Standard events such as Birth, Marriage & Death, which are defined in the GEDCOM specification, are often treated in special ways by genealogy products. For example, the Birth event Date is used to determine the Age of people.
If you created a similarly named custom event then no product would use it to determine Age.
The standard events such as Marriage, Separation & Divorce may also have special features.
If you changed all your Marriage events to a custom fact then those special features would not apply.
Therefore, I advise you to keep the thousand or so standard Marriage events as they are.
The narrative sentence for those events is presumably perfectly satisfactory in both FH and GedSite.
So focus on just the 18 domestic partnerships and consider the options for those.
The Sentence Template for your Partners custom event seems to be fine:
{couple} became partners {date} < at {address}>< {place}>< {age}>. {note}
Why do you need to change it?
If you want GedSite to treat that Partner custom event as if it were a standard Marriage event then changes are required.
The only way to get GedSite to work that way is to use a standard Marriage event but specially customised.
Since there are only 18 events involved that adjustment may as well be done by hand.
I believe you want to attach Dates to those events so cannot use Marks suggestion of Status: Never Married.
The important aspect of all standard facts is that they have a Descriptor TYPE sub-tag.
The GEDCOM specification gives examples for partnerships where the Descriptor TYPE identifies the 'Marriage Type' as "Common Law" or "Tribal Custom" for example.
The technique for using this Descriptor TYPE and the associated adjustments is explained in FHUG Knowledge Base
Recording a Civil Partnership especially under
Fact Type Descriptor but most of the other sections are important to understand too as they explain the pros & cons of the options.
Just substitute say Partners instead of Civil Union to achieve the effect you need.
It will also need an experiment involving GedSite to check the full effect.
To summarise, set up an experiment perhaps using the Family Historian Sample Project, in which you have couples with no Family facts, with standard Marriage events, custom Partners events, and Marriage events with a Partners Descriptor TYPE. Check how they are handled by FH and GedSite in Diagrams and Reports before deciding which way to jump.