I know this is a year old but I wanted to answer this to allow future help in this topic. I am currently using blender 3.2.0
So what I did was open up a new blender file - KILL cube, cam and light. SWITCH editor type to video sequencer in viewport, ADD > movie, and SELECT "animatic file.mp4" & ENTER. In Sequencer SELECT audio clip portion of your animatic file and Press N to show Sidebar (if not already shown). In Sidebar, under the strip tab, CLICK on Sound option and CHECK display waveform. Collapse the Sound option, and under Time option, make sure your clip start is at frame 1 (or whatever suits your need). CTRL + MIDDLEMOUSE click and drag up or down to zoom clip and SHIFT MIDDLEMOUSE & DRAG to pan clip as needed. On the upper right of video sequencer editor CHECK thumbnails option under show overlays for sexy. CLICK on the View option on top of sequencer and CHECK the Channel option. LOCK both audio and video channels on left. You can rename channel as you like.
Now SPLIT viewport area and SWITCH editor type to Timeline on newer viewport. CLICK on Playback option and SELECT Sync to Audio next to Sync option & CLICK on checkbox next to Scrubbing option. ADJUST End frame to match your needs.
SHIFT A or CLICK Add on 3D Viewport & ADD Empty > Image. In Properties editor type, SELECT Object Data Properties tab and under Image section, drop down, SELECT and OPEN your "animatic file.mp4". In the same Image section ADJUST your Frames so that it matches your needs. Make sure Auto Refresh is CHECKED. In 3D viewport hit R, X type 90 & ENTER to rotate your animatic to view. Hit Numpad 1 for front view. Hit G, Z & and Move animatic up so that the base of it lines up with 3D floor (Hit N to bring up toolbar and CHECK Lock to 3D Cursor for help lining things up). In Outliner RENAME Empty object into "Animatic" and CLICK on disable selection in viewport on right restriction toggles to prevent accidental movement (Click on Filter option on top of Outliner and turn on this, if you do not see the toggle).
Now make sure you are still on front view and hit SHIFT A, ADD Camera object. Hit G, Y and Move camera back to be in front of Animatic. Hit Numpad 0 to go into camera view and hit G, Z and move camera up and G, Y to move cam closer. Hit G & Move until camera frame matches animatic frame. Go to Output Properties Tab in Properties Editor and make sure Resolution and Frame rate matches your Animatic's format.
CREATE another Area and SWITCH editor type to Dopesheet and CHANGE Dopesheet context option to Grease Pencil. MAKE sure you are on frame 1 on timeline. SHIFT A to ADD > Grease Pencil > Stroke. In 3D view GO upper right to Viewport Overlays toggles, CLICK on dropdown arrow & CHECK box next to Canvas option (down at Grease Pencil section). You should now see a Grid over your Stroke. GO to object data properties and go down to Canvas section and ADJUST Color, scale etc to your needs. With Grease Pencil object selected, hit G, Z and MOVE up Stroke until base of grid is right on top of 3D floor.
SWITCH to Draw Mode and erase Stroke. SCRUB through animatic and begin storyboarding. Make sure Autokeying is active in playback timeline. If animatic is way to bright SELECT animatic object and in Object Data Properties tab, CHECK Opacity and ADJUUST as needed. GO up right in 3D viewport and CLICK dropdown on Viewport Shading & under Background SELECT Viewport option and CHOOSE brighter color to further dim animatic.
This is how I did my setup and hopefully it still is of help to you. Hope you never give up! Love!
