Is the Mytek Brooklyn Bridge or Lumin D2 good buys in 2021?

If you are leaning towards Lumin, I should probably share my less than stellar experience with their app.
Almost every time I use it, it will back out of Tidal, or the Lumin app will crash altogether. Yessss, it’s updated, unplugged/rebooted (network & streamer), hard wired connection, new switch, router, NAS, modem, Supra Ethernet cable blah blah. No change. I have Bluesound equipment in another system (on the same network) with absolutely zero problems- even when I use Tidal through their app. Opinions may differ, but for me, Bluesound’s app is where it’s at! Reliable and easy to see info with an intuitive user interface. Lumin’s app is nowhere near as reliable as or intuitive as Bluesound’s app (for me). I’ve tried different, older iPads, newer iPads, iPhone 6, new iPhone12pro: nothing has made a difference. Lumin app does not work well with Tidal (for me). I went so far as to purchase a 2nd Lumin streamer to see if that was the problem. No change. Im not a horizontal-scrollin’ Roon fan at all, but I might be soon. I’ve just about had it with the slow, laggy and crash-prone Lumin app. 
YRMV

I have used a LUMIN D1 for 5 years - it ran flawlessly - it uses the Wolfson DAC / got constant updates on the software  on the internet from LUMIN- when MQA came out my D1 could fully unfold the files at no extra cost - definitely heard an improvement in my sound. I stream Tidal HiFi .  I sold the D1 5 months ago and now I run the LUMIN T2 in my main system / it runs a Sabre DAC - bigger sound stage and more detail than my D1. I just added a LUMIN D2 for my second system in the Mancave - love the LUMIN D2 - the Wolfson DAC has a warmer sound - vocals are great with the D2. Build quality on LUMIN streamer / DACs is great - software support is great - both the T2 and D2 are improving in sound as they break in.
just my 2 cents - Happy Listening!

Lumin D2 was a disaster for me. Played Tunein but not Tidal. Would play a song for 2 minutes then have dropouts with either WiFi or ethernet. No  support from Lumin from either online or phone. Other streamers worked fine and in short order. Sent the damn thing back. 

I would not separate the dac from the streamer because of several reasons.  First, internet music nowadays are streamed in much higher bit rate with a lot of information and it's up to the receiving end i.e the dac to be able to process that.  Having all in one streaming DAC would eliminate the need for possible noise in the transmission via the cable.  Yes, the signal is digital but both ends will have to do extra processing of the music stream rather than doing 1x.  Second, the best cable is the shortest cable so doing this, you achieve avoiding signal integrity issues between your dac and streamer.  Processing streaming data from network to music format is done once.  Network is one of the most realizable transmission of signal because it has data and security error handling built in the protocol.  Third, you might save money on having to buy two units along with interconnects.   The PS DSD DAC is probably one of the best DAC for the money to be able to accommodate all these while still provides the flexibility to be used as a pre-amp and standalone DAC. I was looking at the X1 streamer and I came across 1 person who had them both for comparison and yes, the X1 is tiny bit better but for 2x the price, new or used,  and most importantly, X1 can't be used as a standalone DAC.  X1 has the latest and the best processors (2) SABRE DACs that can process 32bit data stream so you're future proof but I haven't seen any streaming service that provides that 32bits content.  TIDAL is maxed out at 24bit for their MASTER subscription.

Another vote for Lumin.  I'm using their U1 Mini to my Ayre QB-9 and thrilled with the combo.