You linked to the thread where I gave the solution to the original author.
Have you tried it? The same solution works for your case as well (even without the “Instance swap” property).
I edited the “LayoutGrid” component and added exposed nested instances properties for the “TestItemGrid” and “FormsCheckboxList” components (to change variants at the top level), and added text properties for the “TestComponent” and “FormsCheckbox” components (to preserve overrides when swap nested instances).