Enough procrastinating, today I finally took the time to get my Analogue 3D and Summercart64 sorted. I had been playing with it pretty much on default settings, which were locked at 1080p and I couldn't figure out why, and except for updating firmwares I hadn't done anything yet about minor tweaks. I now have the all cart images in the Analogue 3D menu and all the boxart images in the Summercart menu. That includes a cart image that matches my Kaico SC64, even though it still shows as "Unknown Cartridge." I also set the RTC on the Summercart (for some reason it thought it was still in 2024), and on the Analogue 3D I figured out how to get it to play nice with my older 55" Sony Bravia so it now displays fullscreen, i.e. no black space top/bottom, in 4k on any setting. (needed to enable "Enhanced HDMI" for the specific input channel the A3D is plugged into, or else it would force 1080p only. Just a quirk with my old TV, not the A3D.).
It all looks glorious now.
Then I proceeded to get 14 stars in SM64, and 22 balloons in Diddy Kong Racing.