I have 11.6.0-39U
I see the guide to installing ntrboot on 3ds.guide, I can follow those instructions easily enough.
I have various questions about cfw and the ROMs etc. If there is an existing guide or FAQ for my questions please link me, I haven't found the answers online yet, or maybe I don't recognize them.
If I put ntrboot (does that include luma3ds?, not sure how everything is related), when all the setup is done will it look and function just as it does now? Ntrboot is something flashed to a DS flashcart that allows software requiring arm9 access to work. This includes SafeB9SInstaller which means you can install b9s using it. When you have Luma everything will be the same as before only you'll have access to a lot more stuff
I assume then, as Nintendo updates firmware which you'll need to match to access the store, I'll need to update the cfw as well? I don't know what you mean by access to the store but you'll always be able to update your firmware. If Nintendo pulls some sort of trick that stops the current version of Luma working properly you may need to update Luma, but that's all
And how would GBA/SNES roms work? Would they reside on the SD card inside my 3ds, or on the flashcart? Would they be accessed via the home screen, or from a separate launcher? You will install them like other 3DS games and you'll open them from the home menu Can I use my existing ROMs that I've used on my PC for snes9x and virtual boy advance or would I need different ones that are designed for the 3ds? Different ones, they'll be injected into cia files so you can install them. You can inject them yourself out find versions already injected
My friend has a stock 3ds and dslite that he uses an r4 in and runs the "game" that the ds detects the r4 as, and that has separate firmware for all the ROMs.
For playing DS games on the 3DS it will be the same
I don't mind spending the extra money if it means I don't have to deal with changing the firmware what do you mean "changing the firmware"??? (that's more time and effort than I would like to spend on something I can do in 30 seconds on my computer). What downsides does the dstwo have besides price? Drains your battery when inserted even if not being used