What I’m looking for in a streaming transport

It will probably cost more than $1500 - but the Lumin U1 Mini would meet your requirements. 

There is an inexpensive and simple conversion kit available to change out the IEC so you can use an external LPS with it.  Upscale Audio sells one.

Have you checked the Cambridge Audio streamer?

I would recommend a used Aurender. Fantastic sound, models with all variations. I personally would go without transport. Putting all the funds into the streaming function.

Pro-Ject Stream Box S2 Ultra. 2 USB ins with more juice to run drives than BluNode. Everything but LPS. John Westlake designed it to not need one. I got a Teddy Pardo PSU for mine before I read this. No noticeable improvment.
I dropped <$1K on it, all in. Including an Atlas Element USB cable  .
2 pages of the included manual, up and running Qobuz and SSDs. 
It sounds as good as my Allo/Shanti, which requires some work, but has been reviewed better than Lumin.

"Pro-Ject Stream Box S2 Ultra" 2 USB ins with more juice to run drives than BluNode. Everything but LPS" I've considered this

"I personally would go without transport" Not referring to a physical drive; some manufacturers refer to streamers lacking dacs as a transport- the nomenclature is difficult because there is no standardization between manufacturers 

"I would recommend a used Aurender"- You're not the first one who has suggested this route 

"Lumin U1 Mini" on my short list and I see Teddy Pardo has an external LPS for this too but with an external LPS we are at $2500 and at that level there are other options in the ballpark and that's more than a Zen Mini Mk 3 with LPS

"Cambridge Audio streamer?"  I've not really looked into Cambridge

So far, there is no commercially available product that is 100% in accord with my wish list.The unfortunate thing is that the best streamer I ever heard was a 5 figure Naim streamer. Liked it better than an even more costly Linn streamer. 

One thought that keeps popping up is just get a Node for $549 (or maybe a new old stock Node 2i for even less), use it for a year or two, and see what is available then. I would not be using the Node's internal DAC as I have a more than adequate DAC already. 

Ante up for the Auralic G1 streamer. It includes a 1GB playback cache with reclocking - dejittering, LAN-level-performance WiFi and makes for an outstanding feed to a DAC. It makes every DAC I've tried with it sound better than prior. A bonus is the Lightning DS iOS control software sounds better with it than Roon and provides a clear, direct interface.

Phil

Used Bryston BDP Pi or BDP 2. Comes close. Saw a Pi (with SGC LPS) and a 2 (with upgrades) over on usaudiomart the other day. Manic Moose is certainly spartan but it works and the SQ is up there with the best. My Pi has been discontinued but I still get firmware updates and support. Both Work stand alone or as Roons endpoint. The perk with those units is you can add a USB DVD drive or Bryston BOT -1 and Manic Moose will allow you to rip CDs to connected storage. I was in my local library the other day and saw an entire shelf of Jazz CDs to check out. Im’a rip them all.

If you don’t mind a little effort in setting it up, a Raspberry Pi is an excellent streamer. Depending how you configure it you can get set < $200. 

I am searching for the same thing. The Lumin Mini seems to be the best choice in the streamer only space, but still too expensive for my tastes. In the meantime I put together a Pi 4 with an Allo DigiOne board and low end LPS, all running Moode. Running it through a Gumby and it sounds pretty amazing. Nice upgrade from my Sonos Connect. I am keeping my eyes open for an even better solution at the right price.

Teddy also has a LPS upgrade for the Bluesound Node.   UI is outstanding.   I use mine for streaming and also for transport of .flacs stored on an NAS hanging off the same router.  Internal DAC is OK but will be the  second upgrade.