I hadn’t seen this discussion before. I thought it might be helpful to specify my current set-up as it delivers much of the functionality that the OP wants, today.
I use a programmable split keyboard, the Moonlander. (I’ve had it for years; I love it.) In between the left and right keyboards, I keep a Magic Trackpad. (I have an office and a home-office; I move and set this up in each place every day. I have external monitors at each location.) I use BetterTouchTool Floating Menus extensively. I trigger a top-level Floating Menu with a trackpad gesture. Other Floating Menus are app-specific. And so on with all the utility Floating Menus provide.
For me – an avid trackpad user (I use each hand on it) – the tandem trackpad+FloatingMenus gives me everything I think I could get from a Touchbar.
I have never had nor wanted a Touchbar. I have a MacBook Pro M4.
I have, but currently don’t use, a 15-button StreamDeck. I also have an iPhone. I have settled on a 5-row 3-column grid of square Triggers for my Floating Menus. In time I will program the StreamDeck to have the same basic triggering functionality as my Floating Menus. In time, I will also set up BTT-remote (or whatever it is called) to work the same on my iPhone.
I think those combinations with their broader and deeper software-controlled mutability (because of BetterTouchTool), will better serve me than a keyboard with a built-in Touchbar.