I have just purchased a “low cost” 7" touchscreen here: Écran LCD D 7 pouces, 1024x600, avec support h, angle de vision complet à 178 degrés, pour raspberry PI 3B + 4B | AliExpress
It did not work out of the box, I hade to modify the resolution parameters to have it work.
Here is finally what I used, in file /boot/userconfig.txt:
The screen is given for 1024x600, but unfortunately, the “standard” configuration for this resolution did not work:
hdmi_cvt = 1024 600 60 6
hdmi_group = 2
hdmi_mode = 87
hdmi_drive = 2
This configuration made the screen blink. I don’t know why.
Anyway, the one I chose is nice, given the size of the screen. It is equivalent to 848x480, which is nice pour Volumio WebUI on this 7" screen.
I found all relevant information for hdmi configuration here: Video options in config.txt - Raspberry Pi Documentation
I tested the screen on RPI3 and RPI4. I found it a good value for money. It comes with a set of adapters for HDMI and USB Touch for both RPIs, and a simple but very useful stand.
If this can help.