PrismLauncher
This launcher lets you manage multiple "instances", which have the functionality of multiple .minecraft folders,
- Minecraft accounts are managed globally and can be selected before launching an instance
- As of writing there is no in-game mods that allow you to switch accounts in game, might be because of some of these reasons
- You can specify a java runtime environment, ram usage and jvm arguments per instance
- Since 9.0 there is now an Automatic Java Management option to easily download JREs
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assets, libraries, logs, mods, saves, screenshots, resourcepacks, assets, config files.. everything is separate
- https://prismlauncher.org
- https://github.com/PrismLauncher/PrismLauncher
Modpacks
You can install and update individual mods, resourcepacks, shaders and modpacks from Modrinth and CurseForge
I recommend checking out my simple-mod-pack if you're looking for a 1.21.4 Fabric modpack, I tried combining mods and config options from every performance modpack I could find and added various quality of life mods, whilst keeping too opinionated settings disabled (you can re-enable them in the mod list).
Sharing instance subfolders across multiple instances
Let's say you have multiple instances and a single resourcepacks folder, how can these instances all use the same folder without making copies?
Symbolic links let you do that, you can think of them as a shortcut to a folder that's somewhere else.
You can find the instance's folder path by right clicking it and clicking "folder", which will open it in your system explorer, then you open the .minecraft folder, then the subfolder you want to turn into a symlink, after that copy the path (CTRL+L + CTRL+C), back up files in it/put them somewhere else, go up a directory and delete the folder so you can create a symlink with the same name.
You can create a symlink with the following PowerShell command, the first path provided is where your subfolder actually is, and the second is where to place the symlink.
New-Item -ItemType SymbolicLink "$env:APPDATA\.minecraft\resourcepacks" "$env:APPDATA\PrismLauncher\instances\1.21.4\.minecraft\resourcepacks"
$env:APPDATA is the equivalent of %APPDATA% in PowerShell