On the SEU issue I’ll add in that even in LEO you can still get SEUs - the ISS is in LEO and gets SEUs on occasion. There’s also the South Atlantic Anomaly where spacecraft in LEO see a higher number of SEUs.
As a sibling post noted, SEUs are possible all the way down to sea level. The recent Airbus mass intervention was essentially a fix for a badly handled SEU in a corner case.