I've done it myself, several times.

The cheap, easy & effective way is to just vacuum pump down the old system, and charge it up with the $35 kit you get at Walmart and the like. It works.

The more difficult but appreciably quieter route is to disassemble the compressor and suck the whole system of oil. Then use a "proper" R-134 oil and recharge. I've done this twice now, and it was very noticable the change in compressor noise. But it's a tedious job.

The trucks all would cool poorly if the system was low. Be it R-134, or the older R12. Being ever high tech, I would throw a can in, and be satisfied. The wifes car was/is low enough that I might throw a second can into it.

None of them cool as well at idle as they running down the road. The compressor is spinning faster, and it cools better. But set up correctly, all of them were quite cool sitting in traffic, 100+ degrees, baking in the sun.

Is yours idling up the engine enough at idle to compensate for the load? And does yours have that control unit that frequently cuts the compressor out at idle? The earlier Toyota trucks were infamous for having no cooling at idle because of this.

I've played with my fancy manifold, read the gauges, made notes, and it got me nowhere but more confused generally. Bottom line for me is, is it cold? I don't care if my gauges are reading wrong values, is it cold?

I have noticed that the R-134 converted systems are more sensitive to a loss of refrigerant, and seem to require a higher overall pressure than they are supposed to need. But that's observed without a whole lot of experience.