can i get a significant dac upgrade over my Lumin d2 for 2k?

yes by all means, go for it! ....and report back!  🤣

Yeah, the Gustard R26 seems like a damn good bet although I prefer using a separate active preamp. 

i just tried Topping's new preamp and was not too impressed with the unit

noise floor was very high in my system

at the moment I'm getting great results from my Lumin straight to power amp

Lumin directly to power amp is an excellent choice if using the LEEDH processor in the app.

I very much doubt that you will get anything more than minimal to negligible. In essence, a likely sideways move at best.

In my experience, the lower limit “cut-off” pricing threshold of where the contenders start to materially separate themselves from the bevy of pretenders, is going progressively north of $3K ( Pre-COVID pricing strata).And that does not include the added matched cost of a separate stand-alone stablemate  DAC.

That is where the upgraded build quality and resulting audio performance-step-up follows the price-point upward $$ curve.

A simple example is the improved design and better build with the implementation of robust linear power supplies.( either external or fully isolated built-in)