There are three options, DSiWare hax (requiring a second already hacked console), ntrboot (requiring a compatible flashcart), or a hardmod (requiring soldering experience and equipment). There's unlikely to be anything before EOL because it is now so easy. I don't understand your last sentence of this question. Why don't you "trust" flashcarts? Are you afraid they'll become sentient and rob you? What does 128GB have to do with anything?
I don't know what RMA is or what you are asking here.
I mean, if i need to use warranty in case of faulty console. Can i undo cfw and anything what i did before sending 3DS to Nintendo for repair?
I dont like R4 because its another hardware part which can be damaged. That button must have some lifespan. And i thought, you can use maximaly 32GB card And i would like to use 128GB. I cant find any in Czech republic.