try using transaction saved search and main line false

look into getting additional suitecloud plus license – will speed up processes by allowing to have more governance limit

if it is a sublist field then you can easily make it mandatory using a simple workflow – lookup in suiteawnswer on how to do it – #40492

I have loads of audit related searches for ITGC and auditing in my 2019 Suiteworld presentation use the link below
2019 Suiteworld

also make main line false

Gatekeeper is a great product I just had demo couple of weeks ago. But it might be an overkill for only 400+ contracts. You can use CLM(Contract lifecycle management) like PandaDoc or Concord or ContactSafe might more cost effective. Integration is quite straightforward since all you need are the start and expiry dates from CLM along with contract status into NS. Note all of them also comes with electronic signature too.

add a dynamic field in the subject line

This doesn’t make sense, how can you apply prepayment to Bill Credit? Bill Credit means that Vendor owes you money  which you can apply to their invoice, which in other word same thing as prepayment which is applied to invoices/bills .   Same principle is true for order to cash process too – here it is customer deposit.

try using the SuiteAnalytics workbook where you can use the multi join option – you should able to drill down further.

you might be able to have a workflow with saved search for serial numbers and throw an exception if the serial number exists in the search results. Just theory though – should work.