Can a off the shelf DAC, if engineered/implemented good, sound as good as a custom DAC?

There’s more to DACs than just different chips and fpga. There are amazing sounding chip dacs and not so amazing sounding fpga dacs in the same price categories. The analog output section makes a huge  difference. There is a reason why different dacs with same chip sets don’t sound the same. I wouldn’t make too many assumptions about what a particular dac sounds like just based on chip without listening to it. 

@dman777 "So my question is... if for instance, the all in one DAC streamer Sim Audio Moon 891 which uses a ESS DAC chip... can it sound as good ..."

The short answer is yes, it can sound as good. Whether it does or not is a much longer discussion with your subjective tastes as the ultimate deciding factor.

Flagship DAC chips from AKM and ESS are like new high performance crate engines from an auto manufacturer (e.g. Ford Coyote V8). The implementation, meaning the tuning, chassis, and suspension is entirely the work of the builder. The engine itself is capable of world class performance in the right hands, but how well that capability is utilized and supported is a product of engineering, and if you enjoy driving the end result enough to buy it is another matter altogether. 

The good news is you should be able to negotiate an evaluation period for anything in this price tier and take the time to decide for yourself.

DAC chips are pretty well refined in 2026 , it is their implementation but more importantly the analog stage that makes a DAC sound good.  

@dman777 

The job of a DAC chip is to convert digital samples into an analog signal. I believe that most modern, widely used DAC chips are already extremely accurate, so audible differences often come more from the surrounding circuitry than from the chip itself.  

In other words, assuming the audio system is properly matched, it is usually everything before and after the DAC chip INCLUDING the power supply, clocking, filtering, and output stage, etc. that makes a DAC based component sound great.

Look into the top tier SCHIIT DAC. You will be astonished.