Some points I would consider:
If ,as you say, the aux fan does not come on with
a/c, that is an indicator of slightly low freon.
[not enough high-side pressure to trip switch]
Try jumping the switch.
If fan comes on, pressure is low/or bad sw.
If fan does not , possible fuse/relay etc.
The 107C you mention is not an a/c fan switch. It is the engine temp coolant high speed fan . Changing the a/c pressure sw has nothing to do with the 107C cut in.
As stated, 134 conversion results in poor cooling compared to 12. However , there is also the remote possible that the system was not properly evacuated and has some air in system. I mention this because you have stated that a tech said you have good pressure readings. Good readings and poor cooling will result if this is the case.
I am also assuming that the first tech stated low fren after looking at the sight galss. Check that first.
The difference from poor cooling to adequate can sometimes be just a few ounces of freon. That would be my first try.