Configuration

To restrict resolution and refresh rate while playing videos, you can use the configuration option in System -> Settings -> Display -> Whitelist. You can then whitelist a set of acceptable resolutions and refresh rates. This is very useful if you want to play 4K videos at a lower refresh rate (for instance because your HDMI cable does not support 4K correctly).

Retroplayer

Kodi (starting with version 18) includes Retroplayer, a game player based on retrolib. You can install normal arcade ROMs (from MAME, Sega Genesis, SNES etc) and directly play them from Kodi (you only need to install the required retrolib cores as Kodi add-ons).
It uses the standard file interface of Kodi, which works very well but can be cumbersome for a large collection of games. Several add-ons can be installed to better manage your game collection, such as Advanced Emulator Launcher (AEL).
Play a game just as you would play a movie, then it will ask you which core to use for this ROM. Note that for ZIP archives, the default action in Kodi opens them, so you need to bring the contextual menu and choose Play rather than use the default "Enter" action.
There is even an add-on named Internet Archive Game Launcher (IAGL) that will automatically download ROMs from the Internet and allow you to launch them.

On some cores, like the Sega Genesis one, multiple controllers are not supported. This is a strange issue as it works perfectly well on Retroarch (on same LibreELEC distribution), and on some cores it works as well on Retroplayer (like the SNES core). But this issue is a blocker for a lot of games.
It seems there are no add-ons for the MAME controller profiles, so controllers are not auto-configured on Retroplayer when using the MAME cores (which is a pain). This also does not happen with Retroarch on the same distribution.

LibreELEC is a full Linux based Kodi distribution. It works on x86 but also ARM, it has for instance a Raspberry Pi image available. It works very well out of the box and is extremely easy to install and configure (5 minutes and you will be playing videos on Kodi).
On LibreELEC 9.2.6 and a Raspberry Pi 4, EmulationStation does not seem to work (yet).
You can install RetroArch on a LibreELEC distribution easily (via gamestarter KODI add-on repository), but I ran into several issues (default Ozone theme does not work and must be changed to XMB, framerates were very low). For now I found using the built-in Retroplayer works better, even though it does not have as many filters / options (and maybe less available cores).
PlayStation 4 controllers via Bluetooth are well supported on LibreELEC.
xrandr is not available on LibreELEC, but you can use tvservice which is a similar tool (in particular tvservice -s).