Are there any features that can pop "reminders" or maybe the Sticky Notes feature, such that every time I start a new sales order, and have chosen the specific customer, reminders pop up about that customer?

This is possible by script and in workflows. See: https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_N2743889.html

Another possibility, create a field on the customer record called 'Reminders', have that source to a field on the Sales Order. No script/wf required.