Solution #3 (cont.)

If fixing the existing archives aren't something you feel like doing, and want the easy way out with using another texture pack to resolve all outstanding issues then you can perform these tasks to make it so one of your own creation supersedes Trrigg's changes.

Pack basics

Make a new folder in where your resourcepacks directory for the profile in use is which represents the directory for this new item — it can be named to preference. For populating it, copy the pack.mcmeta and optionally, pack.png from another, existing pack into the root of said directory.

Open pack.mcmeta and adjust any details of the existing file so it represents details of the new pack being made. Since publishing this pack online will probably get you in trouble, there's no need to be fussed about compressing the contents of the archive for easy sharing unless it is to share among friends

Create the following directory paths (. represents root path):
.\assets\minecraft\textures\block
.\assets\minecraft\optifine\ctm\leaves\birch_leaves\orange
.\assets\minecraft\optifine\ctm\leaves\birch_leaves\cherry_blossom

If you're feeling ambitious, cherry_blossom can be softlinked from orange should your filesystem allow it so all you need to manipulate is orange.

Green Birch leaves

From Stay+True_1.16.zip, navigate to \assets\minecraft\textures\block\green_birch_leaves and extract them to your pack, at .\assets\minecraft\textures\block.
Copy those three files to system clipboard and paste them in .\assets\minecraft\optifine\ctm\leaves\birch_leaves\orange as well.
If you made a separate directory for cherry_blossom, rather than symlinking from orange then go up one level, enter cherry_blossom and paste there also.

Should you want, rather than copying files to assets\minecraft\optifine\...\orange, those files can be hardlinks or softlinks, which means rather then nine files in use, it's really three with three links if you venture to use as few files as possible.

In Minecraft's Resource Packs dialogue, With your pack on top of Stay+True_1.16 (should it show — if it doesn't then your pack.mcmeta file is invalid and needs resolution), the properties files from its path at assets\minecraft\optifine should be used, but the textures from your pack in ...\ctm\leaves\birch_leaves will supersede its copies.

No Bushy Leaves

To resolve, one additional file is necessary. But it has to come from Minecraft.

Open any of the available directories in %appdata%\.minecraft\versions (preferably, latest vanilla build, i.e. 1.16.4) and access using an archival tool (such as 7-Zip or WinRAR) to open its containing .jar file. Navigate to \assets\minecraft\textures\block and extract its oak_leaves.png file to your pack, at .\assets\minecraft\textures\block.

Place ST_NoBushyLeaves above Stay+True_1.16, then your pack above both. The inclusion of oak_leaves.png resolves the issue of Stay True's oak_leaves.png file being a 100% transparent file, which means oak trees will render correctly when electing to remove the bushy textures.