I'm using your strategy to increase our benefits, but in the meanwhile found a sorta gotcha involving our Medicare payments. While we're waiting for my wife to turn 66 to file for her FRA benefit (her's is much less than mine and I will at that time file for spousal and wait until I'm 70 for my benefit and then change her's to spousal) our Medicare Premium has increased from $121.80 to $134.00 because we do not currently receive Social Security benefits to have the premiums deducted from. As soon as she begins benefits (and thus my spousal) we will lock in the rate at $134.00. Makes me shake my head.
I agree that the Medicare 'hold harmless' provision seems unfair. I guess it made sense to Congress when they passed the law, but the result is that it helps some people and hurts others with regard to their Part B premium rates. At least, temporarily. Future Social Security cost of living increases will likely level the playing field at some point.
In any case, it sounds like your filing plan is a sound one based on the limited information given in your question. The 'hold harmless' provision is a frustration, but would not be a cause for amending your filing strategy in my opinion.