Update a Lookup field by automation

I want to be able to update a lookup field based on a contact's action/field updates, but I can't find a way to achieve what I'm looking for, as lookup fields don't appear to be updatable via actions or workflow rules.

Background:

We have 4 segments in our business, each offering B2C activities that are supported by 3rd party contractors, who have restricted user accounts in the CRM, along with one 'top-tier' user account (me) that has visibility across all departments for management
For data security, record access for each user is managed based on the customer area of interest, denoted by a custom picklist field in the contact record, and a 'user access' lookup field.
(i.e. A user working for Dept. B can only see contacts and deals in relation to Dept. B)

However, as you can imagine, we have customers who interact with more than one of these activities over time, and we need a less manual way to keep these records up to date for user access.

At present, we have workflows in place that update the Activity Interest field on the contact record when any new deal is created. When a deal is created for an existing contact but for a 'new-to-them' activity, we have a workflow that updates a tick field denoting them a multi-activity customer, and reverts the contact ownership to the top-tier user, who then manually updates the user access lookup field on the contact.

Is there a way to update the lookup field to 'match' the area of interest by function or workflow?

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