I also struggled with the Linker error you mentioned:
The solution that works for me is to change the LoadingPhase of the ModelViewViewModelBlueprint module from Default to PreDefault. This can be done in the ModelViewViewModel.uplugin file found in the engine folder: UE_5.4\Engine\Plugins\Runtime\ModelViewViewModel\
I found the solution in a thread about the CommonUI: Common UI's loading phase breaks assets - #2 by ElThundercloud
Edit: I need to correct myself: the solution I explained above only worked partially. The linker error was gone and the widget remembered the viewmodel I assigned to it. But the editor crashed on startup whenever I bound a property to a view.
My Gamemode tried to get a reference to the MVVMSubsystem for the creation of its class default object. It turned out to be the exact same issue as described here. That gamemode class created a reference in its ctor using FClassFinder to get my BP_PlayerController. I removed that reference in the c++ class. Created a new blueprint deriving from that c++ class, and set the reference there.
In the end, everything was layed out in front of me. It was just a pain to combine all the pieces correctly. Especially, because this Gamemode was not even used in my current map. Loading Debug symbols and setting a breakpoint in FLinkerLoad::VerifyImportInner line 3489 where the UE_ASSER_LOG reports this Linker Error provided me the decisive clue.
Big thanks at Chonker5.