>why the machines don’t just send a bunch of Terminators back in time to kill Sarah and/or John Connor.
I mean, they're machines, not gods.
A.) They have limited resources. They're fighting a war. Same reason we didn't just send a billion men to Europe against Germany during WWII.
B.) By the time they built the time machine, the humans had already won. This was explained in the first movie (and explained how Kyle came back) and kinda touched on in Genesys - sending the first Terminator back was a complete hail mary pass. Two just implies that the time travel happened at the same time, while three never really explains... well, anything. Four doesn't use time travel, Genesys... well, is fun, but don't logisize it. And the TV show... oh, gawd, you should watch it. They actually DO send back a bunch of Terminators and Resistance fighters. all through the timeline, each trying to make small changes (see the next point) to the timeline to help their respective sides.
C.) Screwing with time travel is bad, yo. Granted, there's a bit of self-fullfilment prophecy built into the Terminator lore (The machines going back gave us the technology to create the machines, Kyle going back gave us the seed to create John), but the machines don't really know this. What if their meddling in the past ends up tipping the humans off on what's to come and we completely prevent Skynet from ever happening? If you're going to alter the timeline, you have to use surgical precision, not blunt force. Something something butterfly effect.
So, there. Don't question Terminator logic. Either, it's explained well within the lore (1, 2, TV), it isn't explained at all (3), or you're just going to have a bad day (5).