Since you used mineral oil, evacuate the system and charge with R12 IMMEDIATELY. R134a requires POE or PAG oil, it is NOT compatible with mineral oil, you'll burn up your compressor in very short order.

Your other option is to evacuate the system, flush again, and refill with PAG or POE and use R134a.

There are many reasons why the A/C may not be blowing very cold. Worn expansion valve, restriction in the system, dirty coils, dirty fan, recirculation flap not functioning, monovalve stuck open....

You also didn't specify how long it takes to cool off. If you start the car and drive away and expect ice cold air, you're in for a disappointment.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]