SAP S/4 HANA Upgrade on-premise Test Planning For Finance Users

Hi,

I'm looking for any guidance or reference articles for SAP upgrade test planning for Finance business users. Currently our organization is with S/4 HANA 1511 and we're in mid of assessment to upgrade 2020 or 2021.

Currently I have SAP HANA upgrade Why, How & Best Practice document as reference but as for Business users we don't know where to start yet.

I'm looking for

1. Lesson learned & journey reference for Technical upgrade then to Functional Upgrade
2. Test planning. We understand that we need to test as much as possible. But with time constraint working with external vendor, want to know key test cases and considerations for FICO, MM & PS in sandbox & QA test planning.

Thank you!