AC barely cooling

Try to turn the front of the compressor with the engine off.  The pulley always spins with the belt, the clutch engages it to the compressor via the flattish looking plate in front of it.  That is the part you need to check to ensure that it turns.

Your pressures look vaguely normal when off.  That doesn't mean that it is overcharged or undercharged, just that the charge is at least high enough that there is some liquid in the system.  The refrigerant will boil inside the system until the gas pressure raises its boiling point to where it finds equilibrium. So, the static pressure is dependent on ambient temperature.

As you can see, the nice thing about these two refrigerants is that the static pressure in PSI is very close to typical ambient temperatures in Fahrenheit so it's easy to remember.

Excessively high static pressure generally mean that there is air in the system, which causes inefficiency but not outright inoperability.

I didn't try to turn the comp with the engine off, but it definitely was engaging and spinning when I pressed the ac button to 'on'. And stops when ac button is off.

Just surprising to me that now turning the ac on causes only the tiniest blip of the needles, and revving the engine no longer causes the needles to move at all.

I'm out of town for over a week, so will pick back up on this after I'm back.

Took a while to get back to looking at this.

Did sort of a re-baseline today:

Ambient temp about 72 degrees

With car off: compressor nose spins by hand with little resistance, both low and high side read about 68 psi

Start car with AC on high/recirc:

Condenser fan comes on with AC

AC clutch kicks in - compressor nose spins with pulley/belt

low side reads about 62 to 65 psi with the needle moving rapidly in that range

high side reads 75 psi needle steady

no discernible cooling below ambient from the vents

Again I detect some slight burning smell

Seems like the clutch may be slipping? Compressor maybe turning slow, not building much pressure?

If not that - compressor failed? Other possibilities or things to check?

changing the cabin air filter and and making sure the heat exchangers are clean and breathing always helps the system.

Sounds like the compressor just isn't compressing.  If the clutch is slipping, it's usually because the compressor is bad.  If you can spin the compressor freely, it doesn't sound like the clutch is likely to be slipping, though.  I suspect compressors fail in ways where they don't totally lock up (even though a total lockup is what usually happens to me). If it were me, I'd probably replace the compressor (and clutch), dryer and orifice/expansion valve.  

What does your cheap pressure gauge say the low side pressure is? Does it agree with the Hammer Store gauge? 

Are you opening the valves at the blue and red connectors at the end of the hoses? Most of the newer manifold sets don't just clip on, you have to open the valve after they are clipped on. 

Can you post a video of what the compressor and gauges are doing? 

I have had issues with Hammer Store gauge sets not allowing refrigerant to flow into the system. It may be worthwhile to return the HF set and buy a better-quality kit. 

There are two types of refrigerant cans on the market. The old ones were puncture cans. The new ones are self-sealing. The valves are not compatible. Make sure you have the correct valve for the can you are using. If the can isn't getting cold, it's not flowing refrigerant. 

Do not turn the refrigerant can upside down when charging the system. Depending on where the low side port is in the system, you can slug the compressor with liquid and damage it. You can speed up the process by shaking the can. 

Assuming the manifold is hooked up correctly, the pressure readings suggest you likely have a damaged compressor. It could be it got a shot of liquid when you opened both manifold valves at the same time and broke the valve set. 

As old as the system is, if the compressor is dead, I'd put a compressor, receiver drier, condenser, orifice/TVX, and every O-Ring in the system in it and start from new. 

It's not hard to do and you can rent the vacuum pump from the local auto parts store.