Does a DDC make SQ of all basic streamer roughly equal?

No a cheap streamer please remember the 4-1 ratio 

which I learned owning a audio store in a $3k streamer only $750 actually goes into theProduct including packaging and fantastic stock power cord ! 
the ddc has better filtering and like the Hermes a High quality clock. 
a separate dac is a must if you truly want the best sound , 

if you have a newer modem - router combo go to Linear Tube Audio

they make a great Zlinear PowerSupply and comes with-a  very good DC cableto go to the router ,a 3.1 docsis is the latest I bought the motorola 8702  much better and faster ,this linear powersupply made a big difference in fidelity

and Ethernet switch upgrade the SW8  is a excellent buy under $600 with a OCxo over controlled lock,and linear power supply , on both at leasta decent powercord

like Pangea awg14 se mk2 , and decent Ethernet cables ,bare minimum $200 -$500+ much better still ,usb also . I am speaking in doing this no matter what dac,and streamer you buy it counts in lower noise better fidelity. 

@soix thanks for the link! Excellent points and I'm curious to try

FWIW, another member here got an Iris to feed his Pontus II from its i2S input and noticed a significant improvement similar to what I experienced although I can’t speak to the benefits with the Ares12. Here’s a nice used Iris listed at $399, but as a new one can now be had for $550 I’d think you could get closer to $350 if you haggle a bit (if it doesn’t impress you can always turn around and sell it). Either way it’s still quite a bit cheaper of an upgrade than buying a new streamer.

https://www.usaudiomart.com/details/650004682-denafrips-iris-ddc/

Thanks @soix. I agree.

My question was a bit theoretical, but I've had negative experiences with some streamers (quality of the app, needing to regularly reboot it).

Most R&D in entry level streamers is SW rather than focusing on the sound quality feeding an external dac. I own an Denafrips ares12, and wondered if a DDC that provides like i2S comes really close to leaving the field of all the streamers in that  < $1500 price range. I'd have to step up to a much higher end streamer to notice benefits.

The only way to know is to audition and like you said, there are many variables. It's a bit embarrassing to be rocking this Wiim Pro, but I haven't had a single glitch with that product. In 3+ months, it's always just worked with Tidal and Qobuz. After finding something that works, my desire to upgrade is really low.

There are a lot of variables involved here, but it’s safe to say a DDC certainly can significantly improve streaming performance. A lot of it depends on what DAC you’re using and how much it can potentially benefit from using a better connection (like AES/EBU or i2S) and a better clock. In my case, being able to feed my Musician Pegasus DAC through its i2S input via a Denafrips Iris produced a big overall improvement in performance over using SPDIF. I don’t really think saying a DDC can potentially negate or minimize the need for a better streamer is the right way to look at this. Especially with streaming every link in the chain matters, and matters a lot, and going with a better streamer will likely produce improvements over and above what a DDC can do. I think it’s better just to think of the benefits a DDC may be able to bring to your system on its own and then consider every other link in the chain on their individual merits. Just my take and experience FWIW.

I wouldn't recommend a DDC unless you have your room dialed in meticulously, it is much more cost effective to use room treatments and DSP before getting a DDC.