Create Automation that adds value to multi select field if it doesn't exist

The reason I wanted to try and avoid this method is that I need that field to still be a dynamic filter, and I've noticed that summery text fields don't have that ability.

You can make a new field  to use as a DF.
ToText([my combined Summary text field])

Thanks you! this helps!

Having done this method, the dynamic filter lists all the possible values instead of each values separately. For example 

1. US ; Mexico
2. US
3. Mexico

unlike the filter for a regular multi-select field that would just list US and Mexico separately whether or not a record had both values selected. Is there a way to fix that?

no, you would need to use the filter box to search or else search at the detail level - the child table instead.