Gohighlevel.com search by tags and update custom values

I would like to look up by tag and possibly tag combinations a list of contacts. Really the ability to look up based on field attributes of dates being in periods would also be nice. Also looking up a list of contacts by list. Then the list could be iterated and I could do mass actions on them. There are several uses but for the main it would just be contacts based on tags.

The other really helpful thing would be to update the custom values. Custom values are universal or global variables that we can use in automation and insert in comunication. The ability to use outside processes and update custom variables with the most up to date data is incredibly valueable. So use cases would be that I can send updated emails and texts using a loop automation but the information in those emails and texts changes based on updates from the api that are then inserted into these comunications via the merge field of the custom value that has been updated. See api here https://developers.gohighlevel.com/#857cd6cd-2b8f-4fee-9f26-267eda31729f If someone showed me how to use a webhook to post out and update that would be just as good.