I'll make a copy of your comments to have with me when I practice.
Flamenco, as far as I know, has been labeled as 4-finger tremolo only because of the 4 times you hit the treble--not by actually using four different fingers! The same with classical, which is 3-finger tremolo.
One other thing I was thinking: If the i a m i sequence is practiced without the thumb, I think it would be a good idea to pause by one count after the last i, which will represent one beat for the thumb. I tried that by starting with the count of 5 and then 1 for i, 2 for a, 3 for m and 4 for i and 5 for the missing thumb. It gets to be natural without having to count but it's more real. As you say, that flow of the fingers is what needs to be perfected.
Muchas gracias,
Worick