Migrating Mixpanel Client: ID Merge Recommendations Needed

Hi all, I am currently in the process of migrating a paid customer of Mixpanel from a client-side implementation (via Segment) to an HTTP API-based implementation (using GA4 events sent to GTM Server-Side and from there to Mixpanel using Stape's Mixpanel tag template). My problem is the following: The client's project is a rather old one - created in 2014 - and the Identity Merge is not enabled (s. screenshot). From what I can see, the Segment library handles the ID merging logic on its own, and is using some outdated parameters (e.g., $anon_distinct_id, etc.). Currently, I am wondering if I should simply recommend the client to upgrade to the Simplified ID Merge by creating a new project. What is your recommendation in this case? And if you recommend staying on the Legacy ID management project, is there any documentation to be found on how to implement it? Many thanks in advance!