Error Patching SharePoint SE to Feb 2026

Recently we tried a SPSE patch from CU July 2023 to CU Feb 2026. Seems the leap was too big for SharePoint.

The patch destroyed the farm:
1. Took 2 hours to complete
2. Successfully ran PSconfig
3. Sharepoint would just not start. HTTP 500 everywhere (Central Admin, all site collections)

Pinned down the error to:
Source: mssearch.exe y w3wp.exe ( App Pool web apps)
System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.SharePoint.OnPrem.Flighting.SPOnPremFlight.IsFlightEnabled(Int32 flightId)
System.UnauthorizedAccessException: Access is denied. (HRESULT: 0x80070005)
at Microsoft.SharePoint.SPGroup.InitMember()
at Microsoft.SharePoint.SPSecurity.<>c__DisplayClass60_0.b__0()
at Microsoft.SharePoint.Utilities.SecurityContext.RunAsProcess(CodeToRunElevated secureCode)
at Microsoft.SharePoint.OnPrem.Flighting.ECSSPFlightDataProvider.InitializeEnabledFlightsInfoDict()
at Microsoft.SharePoint.OnPrem.Flighting.ECSSPFlightDataProvider..cctor()
at Microsoft.SharePoint.OnPrem.Flighting.SPOnPremFlight..cctor()
at Microsoft.SharePoint.OnPrem.Flighting.SPOnPremFlight.ensureDebugFlightIdsInitialized()
at Microsoft.SharePoint.OnPrem.Flighting.SPOnPremFlight.IsFlightEnabled(Int32 flightId)

This seems to be something related to a OnPrem.Flighting.SPOnPremFlight service. No idea what that is.

So:

1. Do I need to upgrade the SP Workflow Manager first?
2. The SYSTEM account was on WSS_WPG before the update (confirmed it was). Need to remove it before patch?
3. Do I need to enable AMSI before the update?

Had to restore the VMs and all is working fine. Would you think I should:
1. Patch to August 2025 16.0.18526.20508 first and check the farm? (pre-september 2025)
2. Then jump to April 2026?

Thanks for any pointers