basically, the first batch of Switch console Nintendo released had a vulnerability that made them very easy to hack. Those of us who bought a switch at this point lucked out. But after Nintendo realized this problem, the second batch of Switches they released came with a new motherboard, which I think started being released years ago already, and this batch, called "Mariko", is very very hard to hack, which is unfortunately what you have.
But this is to be expected since I think most if not all new switch you find in stores like Best Buy will be the Mariko unit or ones that were patched. So you could either do what Hayato213 is suggesting above like getting a SX core which I have no clue how to do or where to get, or you could buy a used Switch that's part of the first batch, meaning it's still hackable through the easy way.
As far as I remember, back when the Mariko units hit the store, the price of second-hand switch soared because they were the only ones that could be hacked. If this is what you what to do, make sure to verify that the serial # actually belongs to that of a non-patched Switch since I'm sure there are scums out there who will try to scam people by selling you a patched switch while pretending that it is not. Or you could just buy a used switch that's already hacked and running CFW.
So unfortunately there is no easy way around for you anymore or method that doesn't involve spending some money and time. And they are the necessary prerequisite for you to do anything that's not doable on a normal switch, such as extracting your saves to your PC, editing it (which is what allows you to import the Rhea character into your FETH save), using mods etc.
Good luck.