e: I had other text here, but I consolidated it all on another forum, so I posted it here. In short, the below was my issue. I eventually got it to work with the UGREEN adapter which needs my own HDMI cable and doesn't have one built in - the VGA side is built in, but with a P991, I have to use a gender switcher to connect them. In any case, it's working now. I managed to get it all fixed between the time it took me write and post and for it to be approved but I'll leave this up in case someone somewhere eventually has the same problem as me, they can look back at this and be like "thanks, man, cool job"
Basically, I got a Dell P991 and everything is working amazingly well. Dreamcast looks great. Honestly, that's all I've been able to try, but boy, does it look great.
My problem is HDMI to VGA and the sync between, problems therein, whatever else.
RCA HDMI to VGA Adapter
This adapter apparently makes sync drop every few seconds at random intervals. It's never on for more than 5 seconds and is never off for more than 3. Video here: https://imgur.com/a/uaYeqc1
However, at some time, it managed to work perfectly fine. I don't know what I did, but I remember it working when I hadn't actually changed anything other than looking through settings to see if there was something I could change. When I reset the system, problem was back.
I tried changing the power block, power cable, different outlet, and even a different system (PS Classic) to make sure it's not a system-based problem. The PS Classic gave the exact same problems, but the image seemingly pushed everything together (more on this later).
Ringle Adapter
I then got this. It gave me the exact same problems listed above.
Tendak Adapter
Then comes the Tendak.
This one gave me no problems at all for the PS Classic. It worked amazingly well. It even seemingly/somehow changed the resolution of the console itself so none of the words were squished or overlapping - it looked as perfect as it could. However, when I plug in the Super NT, it gives no image at all, and no audio out of its audio port. So, what I'm thinking, is that it needs power, but as its designed, I cannot give it power. It is what it is.
So now I'm just completely and utterly defeated. I have no idea what the problem is and where I could even begin to try and fix it. The Dreamcast plugged directly into the monitor via VGA looks and operates great. The PS Classic using the Tendak looks and operates great. The Super NT just doesn't work reliably on anything at all.
Another user recommended this UGREEN adapter and I have that one coming in tomorrow. Maybe something will change since this one needs an HDMI cable as opposed to a VGA one (but with the P991, I still have to supply the VGA cable and use a gender changer). I also went balls to the wall and got this one since someone in the reviews mentioned a sync problem with their PS4 Pro, so I said "why not."
Just in case you don't want to click the links, the first one is the UGREEN HDMI to VGA adapter and the second is a Jitech big ol aluminum HDMI to VGA adapter.
Any help here, man, please. I'm tired.