I find that pulling the clutch pedal out halfway, letting it drag the engine rpms down faster, then pushing the pedal back in slightly and letting it out as it approaches the right RPM can help a lot. Like catching an egg without breaking it, or receiving a hard pass on a hockey stick...
It's not the best idea to slip the clutch to reduce the engine RPM, causes unneeded wear to the clutch. Honestly, just hold in the clutch pedal until the RPM's are right and smoothly engage the clutch. It's all about timing. As you're learning, you'll probably slip the clutch here and there, but once you get the timing down, there is no need to slip the clutch.
You’re gonna get rear ended one day waiting almost 3 seconds for the rpms to drop 1500.
Which by definition means your RPMs aren't where they need to be for that given speed in that given gear. Wait for the RPMs to drop down or tune out the rev hang. I'll reiterate that I don't recommend using the clutch to physically slow the motor down by slipping it (causing unneeded wear and tear on your clutch) and you won't end up rear ended because of it. Even if it were 3 seconds (which it isn't), you're not going to get rear ended for idling for 3 seconds when making a shift from first to second. Heck, even if it is a full second, you're still not getting rear ended.
