I'm not sure exactly why the Atreus worked for me and TMK currently isn't. The TMK problem is that I am constantly having keys act as modifiers when I intend to just tap (type) them. With the Atreus, I was able to slightly alter my typing to hold down the home row keys for shorter periods when typing, and that solved the problem. I am wondering if the TMK issue is that the setting for IGNORE_MOD_TAP_INTERRUPT is not active in my firmware. According to the page
https://precondition.github.io/home-row-mods, that setting not being on causes similar problems—accidental activation of the mod key when the 1st key is not released until after the 2nd key is pressed. Does TMK include this option?
I am ok with pausing my normal typing style to hold the tap/mod key a bit longer to make it a mod, and then deliberately tap the 2nd key that I want to combine with the mod, if that will help. In other words, I am ok acting more like a hunt-and-peck typist when using the tap/hold keys as mods to invoke keyboard shortcuts. I just need to avoid them acting as mods when I don't want them to, when typing normal text.
I don't care about the ability to hold down a key and get a bunch of them auto-typed (like holding to produce "aaaaaaaaaa"). I'm willing to lose that ability to get home row mods.
Despite my ignorance of the low-level parts of this, I think I can understand the explanations on the "home row mods" webpage linked above, so feel free to ask about things referenced there in helping me troubleshoot. Thanks again for your help!