You are using the MAX3232 chip, because the SoC is a 3V3 device that doesn"t tolerate 5V levels, but the 5V version of the chip may be cheaper and more easy to source.
Note that an easy solution exist, with a simple resistor divider (2K2 top, 3K3 bottom) you can convert the 5V signal to 3V3 for the SoC, and the 3V3 level from the SoC is enough to drive the RS232 driver chip reliable (it needs 2,0V for "1"). Perhaps you can add these resistors to your design. A 0E resistor (wire) instead of the 2K2 resistor can be used when using a 3.3 volt transceiver.
Also, if you create a version that leaves out the breadboard area, you get a much smaller board that fits exactly on the area of the R-pi board without high components, you can then solder the smaller board directly to the R-pi""s pinheaders. That would also take care of the "mounting problem".
Because you repeated the existing pinheader the small board can be permanently affixed.
If you can also put a small I2C RTC IC on the board (8-pin versions do exist) it would make the R-pi a much more complete "PC alternative".