The easiest way to start Dreamcast development is probably still KallistiOS (https://github.com/ljsebald/KallistiOS). There's also the Titan IDE coming out in December which tries to be something like Game Maker, though I don't know how that's gonna turn out.
You'd still need either a Coder's cable, a BBA or a LAN adapter to connect your DC to the PC. I would personally recommend the LAN adapter since it's not slow like the Coder's cable and doesn't cost an arm and a leg like the BBA. Alternatively, you could dev with an emulator like NullDC, though from my experience it tends to struggle with homebrew. Some Dreamcast devs actually use CD-Rs and always burn a new session for testing, but that's pretty much like having a compile time of 5 minutes. Maybe it's possible to dev with an SD card adapter too, but I don't have one so I can't say.
Anyway, thanks for playing and the feedback! I'll have to look into the web version again.