does anyone else worry about the safety of Federal judges?

They're in good hands.

It happens in other countries with kleptocrats and authoritarians in charge

First, court police officers, at every level, are first-rate LEO's who take their responsibilities seriously. They won't let their charges be endangered.

Second, each day, Donald Trump's support is diminishing. Notice how the Republican-led Congress isn't following his "direction." Read the transcript of his interview with Tucker Carlson: even this conservative journalist doesn't believe the president.

The blowback from other judges and the public from a perceived killing would be major since it would signal totalitarianism. Same with killing politicians.

Corruption/bribery on the other hand is possible.