Wow, thank you for all the quick help and useful suggestions – what a lovely community! 🙂
My Dreamcast flicker-issue was resolved after swapping out the VGA cable. Now, the output on my screen is as pretty and crisp as the other consoles!
Thanks to the suggestion from marqs, I managed to get a proper 4:3 aspect ratio on my Panasonic after switching from “Game” mode to “Normal” mode. I even found a moderate sharpness setting on my TV to look great (it’s set to 3 now).
As for my Megadrive II flicker woes, I’m planning to swap out my RGB Scart cable I ordered from consolegoods.co.uk with one from regrogamingcqbles.co.uk – I hope that remedies the flickering!