355 high beam (driver side) is dim

There's a major earth just in front of the luggage compartment fuse & relay holder. I guess you could try cleaning that, but all the lights on that corner of the car use the same earth.
If you're getting 12 volts at the lamp, you would assume the fuses and relays are ok, but I would interchange maybe relay E (high beam) & F (sidelights) to see if it makes a difference.

Maybe there's a wiring problem (a wire hanging on by a single strand). Voltage is there, but the current can't pass.

Where did you check the voltage and what did you use as an earth?

 

Cleaning any and all connection points is usually well advised. I have seen vehicles that showed absolutely no signs of life when the key was turned, no lights, gauges, radio, nothing. And yet the battery post as well as the cable clamp attached to it and the fuse block at the other end of the cable measured a solid 12v+ on a DVM. Removing cable from battery post and thorough cleaning fixed it. The issue is the low-draw DVM voltage reading isn't affected by high resistance at the connection point, but a load such as headlamp is significantly affected with voltage drop. Of course resistance through the relay contacts is also a distinct possibility.

 

Was the bulb "on" when you made this measurement? That would give a more meaningful result (but you almost certainly have an upstream bad, but not open, connection if a new bulb does the same, and you already tried using a known good ground).