Some notes on your notes. SCART can also transmit 480p as I mentioned above. That plus the ease of configuration on the OSSC side is the main reason I recommend it.
SCART is capable of transmitting 480p, yes, but the Dreamcast won’t natively output 480p over SCART, since Dreamcast SCART cables only bridge the pin for SD RGBS, and, even if they did bridge the pins for RGBHV mode, neither the console nor the cables have a sync combiner in order to provide 480p RGBS. Simply, you need something like the BeharBros Toro in order to get 480p RGBS out of the Dreamcast.
Regarding RF output, the NES still had an external box thing, but it also had a modulator internally. Not really sure why it had both, but it did… PCE/TG16 also had internal modulators. But even as recent as the original Xbox had options for getting RF if you really wanted it.
That external box thing is an RF switch, which is distinct from the RF modulator. The RF switch takes the RF signal from the modulator and puts it on the coax line going into your TV so you don’t have to remove the coax cable with those annoying F-type screw connectors from your TV in order to use it.
Regarding region lock, I know that you can play JP games on a US console by way of a boot disc. Not sure about PAL and NTSC, though.
PAL games will also load with a boot disc, but the console will probably switch to PAL output, which is generally unusable on US-market NTSC televisions. Contemporary 4K TVs in the US likely won’t be able to decode PAL, but they might be able to take a 576i/p and/or 50Hz RGB signal; unfortunately, I’m not sure anyone has bothered to confirm that, and it might be a crapshoot.