Vlad I would introduce a MAX Search Mode for Ultimate Users where you take some extra time for a query and include more sources as ones from SerpAPI. Similar to Deep Research, this mode would prioritize depth while taking a delay in delivering results.
SerpAPI is developing a Bulk Mode, which enables multi-topic searches over several hours in multiple regions. This feature could align with the Ultimate plan that currently focuses on AI-focused use cases.
Bing Search is a crucial data source, losing access to it would likely reduce the overall quality of Kagi results.
Integrating SerpAPI as an alternative source can help maintain result quality.