Its long time ago that used my R4i Card ( diffent from yours) for ntrboot. I did twice and it worked flawless following the mentioned tutorial.
Iam not sure atm if it change something, its just a try. U can try dont using the latest kernel for the Card, but the one from 2020. For using the outdated kernel set the time of the 3ds to something befor the timebomb expire.
In that case that your 3DS is already homebrewd with seedminer, wich i understand cfw luma is already installed and runs on it. Did u confirmed that the R4i Card works correctly?
And be sure microsd is correct fileformat with sectorsize.
If flashing the card with ntrboot still dont work, try it on a new day with clear head from the start. Beginnning with formating the micro sd. And beeing sure in every detail for flashing ntrboot on it.