According to Ken, the internal Dac of Airport Express (AE) seems decent if what you have is the newer 2014 model. I am not an expert on measurement. Just to compare several key parameters such as (1) SNR on AE 103 dBA vs 120-130 dBA on the most of entry-level DACs that Topping / Smsl has; and (2) jitter level, it seems AE’s digital out thru Toslink is not as impressive. I do not wish to get into the debate whether we should place the judgement solely on measurements. Since decent entry-level DACs are so affordable nowadays, you might want to just grab one and take a home audition to see if you like them better.
If you want to limit the budget under $500, consider two DACs: Topping E70 Velvet and Smsl Su9 Pro. Very good performers for their price point, good dynamics, soundstage dimensions, bass slams and airy trebles. Detailed but not analytical. The soundstage on almost all Topping/Smsl Chinese Dacs prior to these two DACs in this price point sounds flat. If you like Schiit housesound / tembre, pick a Modius ($200). At these already lower price point, I do not recommend used products because you never know how previous owners treat their gears.
https://www.kenrockwell.com/apple/airport-express-audio-quality-2014.htm#:~:text=The%20AirPort%20Express%20has%20its,the%203.5mm%20analog%20connector.