Photoshop's Frame Animation is EXTREMELY consufing

Hi Tom.  I don't know that I am going to be a lot of help to you, because Photoshop animation is definitely confusing, and definitely not an intuitive skill to master.  Most of what I know I have worked  out for myself, because there is so little in the way of quality tutorials out there.  What I will say, right off the bat, is to not confuse frame animations with video animations, because they are quite different.  I suspect that the best animations are made with video.  It is smoother, and you have sound.  However  Photoshop is not a great tool to use.

As you  suspect, the Onion Skin tool is only available on the video timeline.

I use frame animations, as do several other people on this forum.  I use Photoshop, but have been trying to find the time to learn Animator, which would have some  huge advantages.  You can produce a series of layers, and use Make Frames from Layers.  The trick with this is to use Free Transform Step & Repeat.  You make your moving element, and copy the layer.  Then Free Transform to make any size, angle and or position changes.  The Shift Ctrl Alt T as many times as you need layers.

This  is an example of that technique, and was a tongue in cheek entry for the first Photoshop forum banner challenge.

I learned a useful trick from this.  As the letters get smaller, so do the steps, which gives the impression of slowing as they disappear into the distance.   If you group the layers, you can move and transform them into position and resize.

Photoshop frame animations also has a Tween feature that is very useful.  Mostly for opacity IMO.

A limitation of Photoshop frame animations is that you can only automate one image element.  The banner above has three, so two of them had to have layers manually sync'd to frames.  This is a bit of a chore, and it means you have to plan from the beginning.  Additional elements need to have the same number as frames, but it is all doable with a wee bit of math.  I'm guessing that Animate does not have that limitation, judging from its UI.  After Effects is also very good at moving things around the screen. Both AE and Animate support Ease in and out which makes things look so much better.

Try very hard to get all artwork and layers completed before making the animation, as it can go very pear shaped if you try and change things half way through.  The trick is to select all frames before moving or changing things, but it is inevitable that you'll get a glitch sometime and find something jumping out of position. 

Shortcuts are different, or don't work in the Motion workspace.  One useful shortcut to remember is Ctrl , (Cmd ,) to toggle layers on and off.  If you bring a group of layers in from another document, it is best to turn all the layers off before moving, but Ctrl , gets you out of trouble when you forget.

I'm sure I have forgotten tons of important stuff, but I'll try and add as I think of them.

Good luck

When I paste in an image to use as a background, regardless of what mode it is, PS makes a new project and pastes in the image there.

Please post screenshots including the pertinent Panels (Timeline, Layers, …), because to me it seems unclear what you mean by »new project«.

That's just not a good way of running my project, in this case. I need new frames, not a duplicate of the previous one where I have to erase the whole thing.

Are you erasing content on Layers that are visible in multiple Frames?

I already looked at a different tutorial and used that to create a frame-by-frame animation in the Video Timeline. When switching to Frame Animation mode, the animation I made stays as 1 frame.

»Switching«? Did you use the command »Convert to Frame Animation«?

Screenshots, screenshots, screenshots.

I can only add frames by duplicating the previous frame.

Is »New Frame« not available in the Timeline Panel’s fly-out menu?

To be very blunt: If you are serious about animation you might be wasting your time by doing it in Photoshop.

Photoshop obviously has animation capabilities but it is not actually a great choice for many kinds of/tasks in animation.