I've just been playing with this problem more out of curiosity and have just created a DTMF generator sketch/library which has a more or less a 100% success rate with the earlier DTMF decoder sketch I once created discussed in your previous thread at: Using Goertzel protocol for DTMF and other decoding
Now, the main trick is that it uses an MCU with a built-in 8 bit DAC, namely an ATtiny1614 of which I panic bought a stack during the Corona lockdown period and am slowly working through them.
I'll say that it is "inspired" by the DTMF generator library at GitHub - dilshan/dtmfgen: Library to generate DTMF tones only using Arduino Uno., and I've learned a few tricks from it but, apart from that the code is new. Further, there are significant differences. For example, the whole resistor ladder is dropped and the sine table has a better resolution because it takes advantage in the symmetry of the sine wave and holds a table for quadrant 1 only. Also, since the hardware architecture is different ( AVR v. tinyAVR 1 series) the low level configuration of the timers etc. is different. With common grounds, I connect the output of the generator via a 10k resistor directly to the input of the decoder.
There is some light "polishing" necessary before I publish it but, if you are prepared to go that route and acquire one of these chips, then you can have a "pre-release" version of this code.