Use an LA6500 to increase the current output of an MCP4725
You need a higher voltage power supply for the LA6500.

The table shows the output range (&quot;Maximum output voltage&quot;) when operated from +-15V.  Best case, +-13V.  That difference between supply voltage and output voltage will also hold for lower supply voltages.
The output can't reach the power supply voltage.  It will always &quot;get stuck&quot; a couple of volts below the positive voltage rail, and a couple of volts above the low rail.  With your six volt power supply, the output of the LA6500 can only vary between about 2 volts and 4 volts (give or take a bit.)
You need a higher voltage to get 5V out.
If you need to get below 2V, then you'll need to supply it with a bipolar power supply.
Expressed simply:
The LA6500 doesn't have rail to rail output.