Once you’ve created your level sequence and widget animation; ensure that the widget it being added to the viewport on Event BeginPlay via the Level Blueprint
Then using the Movie Render Queue in the Level Sequencer
Select the config under settings
Add a UI Renderer from the Setting drop-down
You can then choose if the level sequence and widget are combined in the render (checked), or rendered separately for you to combine in other video software yourself (unchecked).
The output from UE will be an image sequence of the combined render, or multiple image sequences of the un-combined render; so long as there’s the same number of frames in both the level sequence and widget animation they should line up as expected