Sorry your car got damaged, but it sounds like it might be a somewhat fortunate thing, presenting the opportunity to really fix your a/c system.

The reason you are dissatisfied with the past performance of your system is simply because R134A is not the proper refrigerant for your system's design parameters. In other words, your system was designed for R12, and until you put R12 in it, it's not going to function to it's designed potential.

Don't waste your time and money experimenting with "substitute" refrigerants when you can just put the proper refrigerant in it and solve your cooling problems. Don't be fooled by substitutes that have "12" in their name, you need the original, the only, R12.