After reading lots of posts on the forum and elsewhere, speaking with Woody @ Holley Technical, bringing the car to the guy who did the engine swap to ensure timing was correct and vacuum advance functioning correctly, etc. I’m no closer to resolving this issue than I was before. If anything, I’m probably more confused so I'm hoping for resolution/clarity.

BASELINE INFORMATION: Rover 4.2 liter V8 (think old Buick Aluminum 215). Possible mild cam or stock cam offset a few degrees (builder doesn’t remember) in a 1970 MGB-GT with 5 speed manual transmission.

Edelbrock Performer Manifold
2BBL Projection Throttle body that started life with an analogue ECU, since upgraded to 2D ECU (534-55), Closed Loop. (This all replaced a 390cfm Holley Carb many, many years ago.)
New style injectors, confirmed in good operating condition.
TPS functions as expected, smooth voltage changes throughout range to WOT.
New, tested, TPS currently installed. (Yes, I threw a part at it.)
TPS is set at .45 volts per Woody’s recommendation vs .63-.65 factory advised. This was done as the first step to rectify the issue. (It didn’t help.)
Potentiometers were set using an MSD Rich/Lean indicator per Holley Instruction manual.
No vacuum leaks that I can find. Hoses are new and visually look in good shape (there only two, vacuum advance & PCV), spraying TB, etc., with starting fluid shows no change in idle.

ISSUE: As stated above and like others, stumble off idle and at cruise, 1,200 – 1,400-ish RPM. Potentiometer adjustments won’t resolve, some changes better, most worse. Like others I can “drive though it” via throttle input. Rich/Lean indicator shows lean (?) if memory serves upon occurrence.

Can’t recall if eliminating the O2 sensor made a difference. I know I did this and assume it didn’t change anything since it's plugged back in now.
No audible exhaust leaks. Headers are tight to heads. Operated fine for years throughout the ECU and injector changes noted above. This started last spring (?), maybe longer ago, but the cure was sidelined due to Covid-19 and affected career change. If I recall correctly, I found a bad vacuum advance, torn diaphragm, at the time of the performance change and thought my issue would be resolved by simply replacing it. That’s when I started down possibly misguided paths to find a solution.