I wasn’t familiar with the boards you mention, but the MAX 10 family is certainly interesting, it’s the first FPGA family from Altera with built-in Analog to Digital converters.
The DECA board you mention is significantly more expensive than the one I am currently playing with, but it has everything you need to built a whole synthesizer, including a 24-bit audio CODEC for high quality audio input and output, ADCs and USB for control and its FPGA is more than eight time larger. With the RAM it also provides, some effects such as echo and reverb could also potentially be implemented, and its Flash memory could be used to store samples for a combined wav trigger and Big Button like drum machine.
The possibilities are endless, but as you say, a lot of that can also be implemented with Teensy or Daisy boards, the latter being particularly interesting with its built-in hardware support for audio.
The learning curve for FPGAs is indeed steep, but not insurmountable, there are more and more on-ramps for software engineers, but I am not too familiar with them. They are very useful to make it easier for AI people to use FPGAs, but that is more applicable to the high end FPGAs that sell for thousands of dollars.
I love the parallel mindset you have to get into when programming in HDLs (being in North America and not in the military, I use SystemVerilog). To this day, I still find exhilarating the idea that the hundreds or even thousands of lines of code that you write all execute every clock cycle, not sequentially one at a time, all at the same time.
As for sharing source code, I haven’t decided yet how much, if any, I will make public. I still have hopes of some day making a semi-commercial product out of some of this work. I have spent years jumping through encryption hoops for my employer to make sure we protected our intellectual property, it’s a hard mindset to get out of.
I admire the generosity of Sam and the Free Open Source community but I also respect people like the Electric Druid who provide interesting product at fairly low cost while keeping their source code private.
There is a huge benefit of getting the community to contribute to improve your products, but there is also an enormous support cost when people start poking around into your code.