That's what the macro above should do
equip BP delay & subtarget yourself or an npc
::press enter::
initiate BP & equip dmg set.
swap into idle set.
edit:
when I had tested it a while back with spells instead of bloodpacts, and instacast gear. it would still get the benefit of the nuke set even though i proc'd insta. I didn't test much though, so maybe it was just a lucky couple swaps.
It's not really an issue with the correct wait times, but the fact that the /equipset command cannot be called more than once per second, and the BP ready time is now one second.
I suspect the reason your <st> macro worked (or seemed to work some/most of the time) is because you left the <st> up for at least 1 second, meaning your precast gear was equipped for a bit before you start casting. This lets you swap to midcast gear almost immediately after you use the ability without hitting the 1 second cooldown on /equipset. If you use <t>, or hit enter as soon as the <st> came up, you'd probably get an error on your second /equipset command and it wouldn't swap.
I gave in and set up Gearswap this weekend. It was painful, and Lua is a shitty language, but it's pretty much a one-time cost and once it's up and running it isn't too bad.