Wait I'm confused now, if you play a legitimate game online using a switch which has previously had a flash cart used it can be banned?

Yes

I'm not up to date on switch modchips but last I checked you needed to solder a flex pcb such that it made contact with pads under a BGA chip

That was only the case for the OLED model, not the LCD and Switch Lite. Nowadays on the OLED model a different method gets used to solder to the Dat0 connection (known as Kamikaze method).

I've done it on all three models and the OLED is the only one that is genuinely difficult to do, yet I was still able to do it without a microscope. The LCD version and Switch Lite are easy enough to do that someone with basic soldering skills can do it