Aiwendil
Missing kernel config is a good guess at this point. If this is your first gentoo install I would recommend using the distribution kernel at first without manual configuration. It makes it easier to setup your system first to a usable state and then later you can still do a manual kernel config but at least have a back-up kernel that works until you get the manual config right.
https://wiki.gentoo.org/wiki/Intel#X_drivers
What do you have in your VIDEO_CARDS variable in your portage settings?
Edit: Also if you configured your own kernel make sure to look at the section above the one i linked if you have everything needed enabled in your kernel config.
Edit2: About the ~amd64 keyword...gentoo packages come in two "branches". The stable and the testing branch. Normally you install a package in the stable version. But if you want you can set the ~amd64 keyword for a package and you get the testing version ( https://wiki.gentoo.org/wiki/ACCEPT_KEYWORDS ). You can mix stable and testing package on your system as you like...but often testing packages will require other testing packages to be installed. For KDE for example plasma 6.8 is currently the testing version and 6.6.6 and 6.7.5 are the stable versions (you can pick which one you want).
I really recommend you read the "Working with gentoo" and "Working with portage" section of the handbook...they explain most of this.
I wouldn't use "=>"...they serve different purposes. But yes, they are all build with the same editor components (ktexteditor). Kwrite for example has no tab support..making it not really useful for editing more than one file at a time...but is simpler to use so rather useful for quick config file editing.
Kate is a full fledged text editor with plugin and LSP support. So it can serve as editor for programming but also as editor for other tasks that require some organizing and multiple files.
kdevelop is a IDE. It includes a lot more tools that aren't strictly editor related (for example tools for easier looking through git patches before submitting them) but it is pretty much only geared towards programming (and there for specific languages like c, cpp, python or php)
Not mention...but kile is a editor specified on creating latex document which also uses the same editor component.