Using a window air conditioner outside and redirecting the cold air into room with ducts

First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.

I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.

Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.

Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.

This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.

One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.

You are correct that the refrigerant is not a consumable. If a system is low on refrigerant, then it has a leak. The technician should have a "sniffer" tool that can detect refrigerant leaks. If they didn't use it last time, then they were irresponsible. Also, refrigerant is expensive so that attitude of "if it gets low again then we know you have a leak" is wasteful of your time and money.

If the compressor in the condenser (the outside unit) is running and the air is not cool, then you are probably low on refrigerant. Frost on the refrigerant is also a pretty clear sign of low refrigerant.

I am additionally concerned about the condensation on your duct work. Is the air handler inside the conditioned envelope of the house? If so, that could explain the condensation you are seeing now but would not be seeing normally.