I appreciate the replies! The black part of the plants doesn't look like BBA or an algae of any type. I can see how it looks like that from the pictures but the black is the green changing color from standard green, to dark green, to eventually dark black and then developing holes/dying. I've had BBA before and is the reason for the SAEs being in the tank in the first place.
I tried to take some better pictures of just the black for you to see. After uploading the pictures, I don't know if it makes much of a difference but when I look at them, you can't see any BBA, just the plant thinning and eventually turning yellow over a really long time period. If this was BBA and with it having been months since this problem started, I would think the rock, wood, filter intake, heater would develop BBA as well; but this issue only shows on certain leaves that start with minor discoloration and then move to more severe erosion.
**EDIT** to reply to most recent comment that posted while I was typing. I can understand the lighting issue but honestly, these plants touched the same height in my previous location and were wonderfully green and healthy for years. If you look at the plants in my tanks, the ones closest to the light are the least impacted. Both tanks basically came with super cheap LED light strips. If you look at the 75 gallon, the leaves impacted are the most shaded. In the 55, it's random across the board but it's definitely impacting leaves further down the plant (middle leaves) versus the leaves closest to the light. Conversely, you can argue that light isn't strong enough and it's the light that is the issue but there are also leaves down at bottom or in the middle of the tank that are perfectly healthy as well.