For older/lower end computers, enhanced-h264ify is a godsend. It blocks youtube from playing VP9 codec which can cause heavy cpu usage on older hardware. It can also block 60fps video as well.
A mouse is a must if you get a dock for it. Some emulator settings are impossible to get to with an external controller. Keyboard is also nice to have.
I would get a powered USB hub for your hard drive, then plug in the hub into your RPI4. The usb ports on the pi cannot handle a lot of power(from what I remember).
On Android and IOS, i like Snikket.
On desktop, Gajim