RePlayOS

There is a new Raspberry Pi based retro gaming OS in development at the moment called RePlayOS.

https://www.replayos.com/

It's still in development and the final v1 is not yet released, but you can download and use the latest version.

What is RePlayOS?
RePlayOS is a Linux distribution featuring a streamlined libretro frontend. It's specifically designed to emulate a wide range of classic game consoles, arcade machines, and computers. The OS focuses on delivering a fast and user-friendly emulation experience, optimized for use with Raspberry Pi boards in both LCD and CRT screens.

Main Features
Support for Raspberry Pi with 64bit CPU models, KMS/DRM and OpenGL ES 3.X (OpenGL ES compatibility mode in Pi3 and Zero 2 models).
Automatic detection of Raspberry Pi model. Same system (SD card) can be used in any supported model.
Support for both GL and Non-GL libretro based cores.
Support for both LCD and CRT screens via DynaRes 2.0 engine.
Support for single or dual screen configurations in both LCD and CRT modes.
Low latency engine with option for zero input lag (does not use runahead).
Low latency audio resampler engine (32ms average).
Support for easy virtual disk engine.
Out of the box automatic core configuration based on Pi model, emulated system, and monitor type.
Out of the box support for more than 700 game controllers.
Support for audio normalization.
Support for external USB/GPIO audio DACs.
Adaptative frontend UI integer scaling, based on system resolution.
Support for frontend UI rotation independently of the running system/game (90, 180 and 270 degrees).
Support for six players.
Support for Favorites and Recent played game list.
Support for Coin-op timer game mode.
Internal arcade game data base for properly displaying game names in frontend UI.
Support for a system halt state (using H key) to facilitate people taking photos of CRT screens.
Support for user X/Y screen position adjustment, gamma and RGB color correction in both LCD and CRT modes.
Support for Kiosk mode.
Support for autostart games on system boot for real arcade cabinet experience.
Support for real GunCon2 lightgun in CRT TVs via custom driver and frontend special functionalities.
Support for RPi5 RetroFlag cases
Ability to filter out arcade games by number of players, number of screens, number of buttons, and screen orientation.
Ability to filter out by arcade, consoles, computers, and handhelds
Ability to boot right into specific system folder for better experience on custom builds.
Ability to enable dynamic colored borders via the AmbiScan feature.
Support for setting the RGB color range (full or limited) for external video DACs.
DynaRes 2.0
When RePlayOS is executed on a CRT TV, CRT PC monitor, or CRT Arcade screen, it makes use of the DynaRes engine which provides the following advanced features:

Native Timings: games are displayed using native horizontal and vertical resolutions and refresh rates.
On-The-Fly Timing Changes: the system is able to make instant timing changes for games that use different resolutions during the gameplay.
Calamity Modeline Calculator: for modeline calculations, DynaRes takes advantage of Calamity's switchres dynamic library for calculating all system modelines.
Interlaced Flicker Reductionː the system automatically applies a linear filter for smoothing the flickering produced in games that use interlaced video modes (slightly reducing the image sharpness).
Software X/Y Position: it is possible to adjust the screen X/Y position via software menu option (no forced modelines).
CRT Profiles: the modelines generated by the system can be configured to better adjust to different CRT types like consumer TVs, PC 31kHz monitors, Arcade 15, 25 and 31kHz, etc.
Dual Screen: you can choose from three different dual screen configurations: cloned image, side-by-side, and stacked monitors.
CSYNC Mode Selector: you can choose different video signal mixer modes: Separated H/V, Csync (AND), Csync (XOR).
DynaRes can also operate in special NRR (Native Refresh Rate) mode on LCD screens, using refresh rates that are either very close to or exactly the game?s native rate. This is distinct from VRR and works on any type of display.

Note: Dual Screen is also available when LCD mode is set.

Custom Cores
RePlayOS provides several utility cores out of the box:

PiBench: a software render core for measuring Raspberry Pi CPU performance.
Screen Test: a simple core for checking CRT geometry and color range, with support for both NTSC and PAL modes (60/50Hz)
Alpha Player: a custom media player for playing video and audio files, with support for many formats.