Sorry the quality is so bad. Hopefully you can see the signal blanking out as I switch between the menu and the running system. The monitor's HDMI2 input notification appears in the top-right each time this happens.
The output resolution is 1080p@60 in both the system and the menu (I tried to show this but it's hard to see the monitor's OSD).
It feels more like the SDL window(?) is being recreated as I switch between the system and the menu.
With the FKMS driver, the monitor never blanks out and switching between the running system and the menu is instant.
Do you care to elaborate on the Video appears a bit degraded part ? I confess I didn't use advmame before, so I don't have a past reference, I just test it out on the Pi4 - the 3.9 version - to see how it works.
Overall, it looks like anti-aliasing doesn't work with the SDL driver. Examples:
Vector game lines are not anti-aliased
System menu font appears jagged
Video Rgb Effects, such as scan2horz do not blend nicely with the game screen.
The setting display_antialias is set to yes, but there is no visual difference when it's disabled or enabled.
One of the draws of using AdvanceMAME is for the clean high-res Vector images, so that's a bit of a miss. But having AdvanceMAME available for rendering raster games like Cotton 2 and Baku Baku is a must-have.
There are also some issues going in and out of the settings menu (Tab). Sometimes, when returning to the game, the screen blanks. Also, gets very sluggish and occasionally hangs. Might not be related to the driver, but different from running this on a 3B+ without Buster.