My source of truth is DB which sends me userid of delegate and leave start and end date in user's data record.
Is there is way that I may update this delegate data to Delegate list against the user as part of incremental recon from DB?
You can stored start date and end date in user’s custom property & use saviynt rest api to create delegations.
you won’t be able to update in database directly as there will be many dependency
Reference for delegate apis