guide VS discord.py

Either way, you're going to need to follow either this guide: https://discordjs.guide/ to get you started with DiscordJS (which is IMO the easiest language to learn it in, also usually the most up-to-date). If you're not into reading (which is not in the slightest optimal if you wanna learn how to program) then there's probably some DiscordJS tutorials online. Just search DiscordJS tutorials on youtube and you're sure to find one.

https://discordjs.guide is the "official" discord.js guide, you'll find a command handler there. I don't know if it will be more or less complicated than the one youre using.

Also do some debug logging to show that the target it actually being read correctly. But moving to slash commands may work better. There is a really good guide out there. DiscordJS Guide

Well, I recommend you JavaScript, it is good for beginners and there's a lot of content on internet. Once you understand and practice the basics, you can use discord.js to write your bots. They have a guide website if you are interested.

but I use the official discord.js guide cooldown example: https://github.com/discordjs/guide/blob/main/code-samples/additional-features/cooldowns/index.js

https://discordjs.guide has a good tutorial on slash commands

Official guide from the peeps who made DJS: https://discordjs.guide/

Read the discord js guide (https://discordjs.guide/). The answer to your problem is that it takes many hours for global / commands to register. The reason I still recommend the guide is because it gives you an introduction to many of these topics. It's better than a lot of youtube video tutorials and such.

discord.py code repository with the code repository for the package,

Discord.py is an API wrapper around Discord. Some examples are available here

You do know it's been in active development now for quite a while? v2.2.3 just came out last week.

Check out this example https://github.com/Rapptz/discord.py/blob/master/examples/app_commands/basic.py

to be frankly most discord bot tutorials are awful, your linked repository is pretty bad too the "official" examples: https://github.com/Rapptz/discord.py/blob/master/examples/reply.py aren't bad at least

discord.py is being actively maintained.

Here is link for repository of discord.py examples. Just go trought code and documentation and find stuff out. Its good to learn this stuff because you will need in some time (for example now)