Thanks, I got into some further thinking, too (always good to get outside the own brain bubble).
This is probably much easier to achieve than the more complex settings you are hoping for.
You are probably right. The new shelf light I built spends most time in solid color, and I would be really happy if I just could adjust brightness with my wheel. Ah, in case you didnt stumble over it, this is my main project. I am open to put all this fancy stuff to the side and focus on a more simplified approach, even this would be a great benefit (kill your darlings, you know ;).
as I’m happy to use the phone app for those
This may be a difference in our approaches. I am not, meaning that I dont want to use my phone more, I want to use it less. Furthermore, as I am going for a retail product, my goal here is to get an MVP that works more or less phone-less (and even some of my tech oriented people surrounding me dont have a smart phone for reasons). As I have written I would be fine with a basic hardware UI with the option to make fancier stuff with phone/computer
like using short press… medium press… and long press
Have you rough ideas about the timings here? With some buttons I have problems always hitting the right timing even with only short and long (and double, thats also a tricky one). With adding medium long press should be…hmm…long. I also have thought about a “timeout” where the “cursor” is jumping back to the T1 menu. One utilization could be: 1. use wheel to dim 2a. when hitting button within X seconds, brightness is saved for the slot/preset and cursor jumps back one level 2b. when doing nothing for X secs, brightness is not saved and cursor jumps back one level. Just as an example how to save a command (e.g. long press) for some simple saving function. But these time based hidden actions should be used very carefully, as these hidden automated actions can easily confuse if not clear or used too often in too many ways. Especially if there is no index.
But it is interesting to think of ways to keep it very simple and intuitive (and more importantly looking simple!), for most users… but allow for more advanced usage for someone trained!
Definitely!
Another way of taking advantage of coded actions could be accessing the expert mode (my config mode) by a cheat code like sequence, or better (bcs. saver) the “hold knob down and move wheel” command (when actions are only executed on release this should be save).
One crucial advantage of such a mode is the complexity you can add without overwhelming a “normal” user, as you can anticipate a pro in front of the light who takes a look at the manual (manual, another important aspect, at least for me 
if she cant remember all the complex stuff. Or you take out you phone/computer and use it. I think thats also part of some user tests to see what ppl actually do and what not/rarely.
so that other advanced modes could be triggered with a magnet in the right place
I really like this idea. As I also aim for “playful control” having an element like a key-card could add some further benefit to the UI. Maybe for sth. like my mentioned “playing” use case a magnet can open such a mode. When having one not so symmetrical you can decode the direction into different modes (I have a fable for blackbox puzzles, but thats another story ^^).
I also thought of a NFC based element that can be used to add new effects (minor updates so to say) to the light. Or you have little cards, nicely illustrated with effects (or sth else), you can attach to the light within a light jukebox mode. Talking about that, having cards could be a proper and elegant solution for all this “effect chaos”, as you have the option to express all relevant info that a user must know.
Centre-push button on the knob to operate a momentary switch. Currently considering differences between those with detents and those without…
Hmm, my intuition tells me without detent you get problems with everything that is a list and not a (at least ordinary) fine-grained scale. Speaking of that, its important to know the hardware limitation, as I dont think you can change more than a few effects in realtime every second without having crushes or lags. Also here the “hold knob and move wheel” function could be of use (instead of acceleration maybe) to either take finer or rougher steps.
For my use case, the led feedback becomes simpler, as colours, or brightness, or warmth can be potrayed using a few leds, to give the idea of what will happen if you rotate left or right at any given point
When already having a knob, one of these 12 LED rings would be a great fit, as their cyclic structures would allow you to put all kind of indices into them (scales, previews, etc…) I will probably go for a dedicated display, but I dont take this into account atm as its a bonus, not a dependency.
Summing it up
We should aim for a normal user friendly “kiosk” mode and an advanced “config/expert” mode. My guts tells me that it also helps me to put all this mind boggling chaos to the side for a moment. Nevertheless I try to take everything into account, as the following other modes should be at least consistent with the basic UIs in some ways.
Though I am no coder myself (I can google error messages and copy code) I think if we get to a detailed and consistent model, we will find the needed help for implementation within these surroundings 
I think about a structure of a working document and open a new one (gdoc or so where you are invited to join me) to bring everything in a form and sketch new diagrams.
Greetz
Kelvin