How to handle mixed sync behavior: Full Refresh first time, then Append only for subsequent syncs?

[leninlal]

I'm developing a custom source connector for Report data and facing a challenge with sync modes and normalization behavior.

Current Setup:

Source: Custom Report API (custom connector)
Sync Requirements:
First sync: Full Refresh | Overwrite (pull all historical data)
Subsequent syncs: Append only (pull only previous fiscal year data)
Using Basic Normalization
Tracking sync state in external database (sync_completed_date)

Sample Code

class Report(ReportStream):
    report_name = "Report"
    primary_key = ""
    
    def path(self, ...):
        if not self.sync_completed_date:
            # First sync - get all historical data
            start_year = date.today().year - 30
            start_date = f"{start_year}-01-01"
        else:
            # Subsequent syncs - get only previous fiscal year data
            start_date, end_date = self.get_fiscal_year_dates(self.fiscal_month)`
        return (
            f"/reports/{self.report_name}&"
            f"start_date={start_date}&"
            f"end_date={end_date}&"
            f"columns={self.output_columns}"
        )

Issue

When I set the stream to "Full Refresh | Overwrite" in the UI:
First sync works correctly (pulls all historical data)
Second sync:
Connector correctly pulls only previous fiscal year data
But normalization still performs overwrite, clearing all historical data
Results in losing data from before the previous fiscal year

Question:

1. How can I control normalization behavior to append data after first sync, even when UI is set to "Full Refresh | Overwrite"?
2. Is there a recommended pattern for implementing this kind of mixed sync behavior (full refresh first, then always append)?

Any guidance would be greatly appreciated!