Did you try the TSA1701 Audio DSP Board?
That's better than what I've seen so far. Looks like actual line level ins
and outs, and not insanely expensive! But no USB audio, just USB power. Yes, I could add a USB DAC to feed it, but that seems a bit Rube-Goldberg-y. (still not bad if it works though...)
You could try the ADAU1702 & friends, simple programming with SigmaStudio (free download) simple to use knit together blocks (no DSP coding knowledge required). I built a sub complete with LPF, equalisation & adaptive overload compression in a matter of weeks and have been very pleased with the results for all the years I have been using it. May not tick every box on your list but definitely worth a look IMOP.
Ultimately, I want to be able to write my own DSP code on my own PCB. The first couple of projects could easily be done with an ADAU and SigmaStudio, but that doesn't get me anywhere towards the later one(s) that have some unique needs that are going to be difficult to find a library for. So while I
could just knock those "easy" ones out with an off-the-shelf solution, I want to use them instead to build up my skills to where I
can roll my own oddball.
I'm a casual Front-of-House and Broadcast Engineer already, so I know what the common processing blocks do and how they sound. Don't need to learn that; just how to make them in software while satisfying my other requirements, and how to manage the chip that does it, in both PCB design and supervisory code.
And I'll probably have some CPU time left over, so it's going to be awfully tempting to combine some other functions into that chip as well. Much easier to do that with a general-purpose chip that happens to have some DSP-friendly stuff in it (compared to the 8-bit world, at least), than with a purpose-designed DSP chip.
Maybe I'm overthinking this. Does
every 32-bit MCU have what an 8-bit hobbyist might call "DSP functions" as part of the instruction set (more than just a hardware multiplier, which is already a luxury in the 8-bit world), and "audio friendly" peripherals? (specifically I2S and/or TDM) If so, then the question probably changes to, "What is the easiest 32-bit MCU for an 8-bit guy to pick up and run with, and then not be limited by its ecosystem?"
That's more along the lines of what I was thinking as, "a good dev board that has the kind of analog audio I/O that I'm looking for." Something to learn a 32-bit MCU on, with USB and line-level audio I/O as the primary focus, then use the same chip in my own designs.