I think the trick here is about keeping the angle of the triangular leaves consistent with rotation. If the leaves are 45 deg when hanging horizontally, unless you plan to apply some gravity on them, then the leaves would remain 45 deg when rotated to make ramps. This mean the edges of the leaves would be straight vertical and horizontal on the ramp tiles, so they would be little squares cut in half, how about giving that a try? Check this out, this is how Surt solved that problem on his Nes-like set which uses similar leaves:
https://opengameart.org/content/simple-nes-like-village-tilesEdit: I took so long to finish this reply that I missed chonky's edit before posting, its a great solution

On the single tile terrain problem, organizing your tiles is hard when you don't look at the whole set together, and when you don't know which type of tile set is the better fit for your task. I prefer to draw tiles as being part of a full
autotile set. Even if you are not using autotiling on a game engine and are placing everything by hand, knowing the full sets and all the necessary parts helps to ensure you can always close shapes made with those tiles. You were missing at least one piece of the 4bit autotile set in the first image you posted:

As you probably know, there are two types of practical autotiling most used in gamedev: 16 piece(4bit), and 47 piece (8bit), each one with its advantages and shortcomings. There are also numerous wang sets and other methods, but these two are the most popular.
The 16 piece one is the most used, and as chonky mentioned regarding unity, this one only checks the sides/edges to decide which tile will fit where. As a result,
if it is used to fill solid areas, it can only render a
minimal shape of a 2x2 square of tiles, and cannot render 1-tile thin platforms and pillars, which is exactly the problem you discovered.
This is a 16 piece set template, notice there is no bordered single tile:

Conversely, there is an alternative arrangement of the 16-piece set which can render 1-tile thin shapes like side-view pipes or topdown viewed fences or narrow walls (sometimes called a fence-type tileset), but then this arrangement becomes
unable to render solid filled shapes such as terrain, because all tiles need to have borders to ensure closing all possible shapes.
This is a a fence-type 16 piece template, it is made of corridors, L turns, T sections, and closed caps. As you can see it has no solid fill tiles:

In the other hand, the 47 piece or 8bit autotile checks for connections at the edges AND the corners, having all the pieces needed to render solid filled shapes AND allow for a minimal thickness of 1 tile, so this is probably what you want to use for your set, ensuring you will never miss a tile to close a shape. The downside to 47 piece sets is the heavier work load of making many more tiles, and the fact that you often have to style your borders in a more compact way so that they can wrap around inside a single tile, and bend into tighter connections.
This is a 47 piece template:

In all the cases above, if ramps are used, they usually fit in the place of an outer corner piece (think of it as cutting off the corners of blocky shape), and are complemented by an inner-corner piece which is styled in way that connects with both flat terrain and with the slopes. If you use a complete autotiling template while drawing your ramp tiles, you can somewhat preview how they will connect with the other tiles, making it easier to design them in a seamless way. Here are templates for inserting ramps replacing outer corners, in both the 16 piece and the 47 piece arrangements:

