Yeah regime content has been a worry of mine for a while. Generally I group large categories together and only break them apart once I have sufficiently gained the skill where I can see the differences. Like vehicles will be grouped together until I start to focus on getting motorbikes really good, then I will study that separately.
I use Google a lot to get the reference images I need, and I try and get an even amount of all different types so that I have a broad input of information. Often I google "Types of vehicle", or "types of flower" instead of trying to get each type individually and I'll find a collage image with 20 or 50 different labelled types. So that makes for a good study page.
I can say from experience that skills definitely overflow from one thing to the next, and to that extent, you probably don't need to study every single thing to the same level, because a lot of the information overlaps. Like after drawing 1000 dogs, the chances are you have learnt to draw fur better, and then when drawing 1000 cats you already have a decent way of drawing fur so you can focus on other smaller details. Same with buildings etc, and pretty much everything.
I would love to hear your feedback on your idea there, it sounds like it could be quite successful.
And that's an absolutely fantastic suggestion for dealing with no motivation!
I would say I'm adding it to my to-do list, but it's actually one of the things I'm doing already! It's a different style to your method but the intention is the same, to get started, and in any form.
Here's how mine looks. The conditions are that I have to draw as fast as I can (at least, not carefully), and as big as I can, so I'm normally zoomed in quite far into the page. Typically circles, triangles and squares are the go-to shapes, although I occasionally draw something 3d. I'm planning on using quite a bit of these as inspiration for my in-universe logos for companies and brands and stuff.
Also the really pointy star pom-pom shapes are a calibration exercise, I draw 4 lines from the center, and then I try and draw lines in between those, and then lines in between those, etc. So it gets harder and harder to keep inside the lines.

I find I very rarely lack motivation to draw, but often I lack the mental energy to deal with creating new sketches, or working to a high skill level like linearts. I've found though, that the feeling is very similar to waking up in the morning - you know that once you get up, in 5 minutes you'll be functioning and doing stuff, but it totally doesnt feel like that's the case when you're wrapped up half asleep in bed. Once I get started on ony of my sketch or WIP pages, even higher level pages like linearts, within 5 minutes my focus picks up and my skill picks up to the required level. But making that jump when you can't imagine having the energy is a huge leap of faith.
It's among the reasons that we can always wake up to go to work no matter how tired we are, because it
has to be done. Generating that level of mentality for a personal project that has no consequences if you don't work on it that one time, is very difficult. Even as high powered as I am, I'm still only 50% efficient with my time at the absolute best of times, and within that 50%, I'm only 50% maximum productive. So in a full 16 hour day, at my very best, I'll produce 4 hours of work, in 8 hours of actual work. Makes me angry lol.
For me, I need more structure when I sketch, so I find it quite difficult to take a random scribble and turn it into a character. (That's probably a skill I need to learn). But precisely because I need more structure, I need to have a structured method in place that I can use when I don't feel motivation. Which is exactly what I have. My default go-to half-finished GIMP page is always there for me to open and just draw anything. And I have my first version of my drawing prompt program pop up automatically by the side, to give me a theme or idea.
I also have a WIP page of just headshots that I have drawn previously which helps to give me a starting point for a new sketch. I usually start every day by drawing a couple more characters in that page too.
Working in your absolute comfort zone is a really good piece of advice. Often artists are recommended to work outside of their comfort zone to gain experience and skill, but on days when you're low on energy and motivation, that's not what you need to get started, you want to work with what comes most easily, and like a warmup, you'll get into it and
then you can work on being outside of your comfort zone. I have a comfort zone page for sketching, because then I dont feel the pressure to draw things unique or special like in my normal page. But ironically I often end up drawing very similar things. It's the tiny action of moving from not-drawing, to drawing, that makes the huge difference.
So long as stylus/pencil is hitting a surface, you are drawing and working at it.
I cannot agree enough, this is the one piece of advice that must be given to beginner artists. JUST DRAW.
I always remember this comic when I think of "just draw"

Sorry for the wall of text haha