@ecto Thanks for the configs, I did some analysis. First: I should have mentioned that after the change of this line to make -j4 PLATFORM="rpi4-sdl2" the amiberry must be rebuilded and installed (sudo ./retropie_packages.sh amiberry), please also make sure you use v4.1.6 of Amiberry and not the current master, just in case you missed it.
From the configs I found these differences to mine:
In amiberry.conf:
Diffs mine (which are also the default amiberry.conf settings) starting with - (red), yours starting with + (green)
You may change them one by one to my settings and retest after each change.
In config.txt:
I have seen you run a high overclocking. Maybe you can disable it for the tests on amiberry. I had different downside effects on Amiberry (3.3 and 4.1.6 when OC over 1950Mhz) (on my 4.19 kernel with fake KMS).
I assume you are using a (4k?) TV for the Raspberry. Can you try with a full HD computer monitor?
E.g., set hdmi_group=2, hdmi_mode=82 or set only hdmi_group=0, I guess you know the switches. --> https://www.raspberrypi.com/documentation/computers/config_txt.html#hdmi_group
And I did not review/change the WHDLoad file. I just let it sit there. You should however update it from time to time (within the Amiberry UI menu), if you haven't done it recently.
HTH