Improving velocity through pulling power is the most taxing change on the periphery. Improving velocity through drag reduction is the least taxing. Manipulating rest falls in between and alternatively fatigues the central and peripheral systems.
Manipulating rest periods allow for longer, and more incremental, periodization. Before you increase target velocity drop the rest a couple times. Then increase the rest period when you decrease the target time.
For 100 swimmers, 50’s at 200 pace does three things: 1) highlights technical deficiencies at a speed approximately 8-10% slower than the 100 pace 2) allows for about double the quality technical repetitions at “near” preferred race pace 3) early in a training cycle stresses the central system more than the peripheral systems.
Program 50’s @ 200 pace after a training break and I would include these at the beginning of the week as the cycle progresses.
It sounds like you’re doing pretty well so far. I don’t think anyone has a good handle on the ideal timeline or expected results when transitioning from traditional to USRPT.