Half an inch thick every morning is way more dedication than I've ever shown my garden, my coffee grounds usually just end up in the trash because I forget about them by 8am. Good to know it actually pays off though, 2 blooms to 14 is a huge jump. That red clay is no joke, so breaking it up with something free makes a lot of sense. I'd be careful with the roses since they can be picky about acid, maybe test it on one bush first instead of the whole driveway row. Worst case you lose one rose and learn something, best case you get a wall of flowers.

The "night and day" thing is where I'd push back a bit. A wet trowel helps the mud slide but it can water down the mix right at the surface. I've had boards look great going on and then cure soft in spots. Give me a dry trowel and a slightly looser batch any day.

20% more sounds a little high unless you were running really wet. More realistic is maybe 8-12% bump on average but still worth it. A washing machine motor with a pulley setup works fine just make sure you balance the load or it'll shake your garage apart.