How to create a two way Sync with CRM Contacts Module?

Newbie creator here (but not to Zoho CRM). I want to create an app that operates on a sub-set of CRM Contacts - only those with a specific tag. I want the app records to mirror the tagged contacts in CRM. I would like it to update when the Creator app opens, and then whenever tags are changed in either location.

The reason is that I want to separate business CRM functions from ongoing client services, and more database room. I run an athletic training facility for teams, so the app will ideally run individual training programs and store historical data while CRM is business.

Can I create the Contacts sync with out of the box integrations via Flow, or do I need to code a deluge function to get the records like Athlete=zoho.crm.getrecords(...)?

If I need to code, is it possible to initially run the function whenever the app opens? I have never done any API work before.

EDIT: I know 2-way syncs are finicky so it may be two separate 1-way syncs. Any way I can do it is fine.