In the interim I assume I'd be able to hook up to them up to my iMac for just playing backing tracks through as well?
I like the Mackies because they are easy from that regard, and the 4" even have Bluetooth to play from my phone. But then wouldn't be as good for recording down the road methinks...
I'm not familiar with the Mackie speakers, so I can't say for sure whether they would work for recording. But they would be great for playing backing tracks, as long as you don't have a raging loud amp.
The main thing about studio monitors is they're designed to have a very full range and accurate sound reproduction, which will help you fine tune mixes, tone and EQ when you get into home recording. So, whether the Mackies can do that too, I'm not sure. I used a set of stereo speakers as monitors for a while, and when I switched to true monitors, it was a night and day difference. A lot of things I had recorded using the stereo speakers for monitors, I would listen to the recordings on another source, like in the car or a pair of headphones, and for one reason or another my tone was junk. Turned out to be that the stereo speakers couldn't reproduce the full frequency spectrum that I needed to really dial in my tone, especially regarding mic position.
The downside to the JBL's, and probably a lot of similar monitors, is that you'll need a USB interface to support them. They use either a TRS or an XLR cable to feed signal to them, and most laptops don't have that capability on their own. Maybe there are some that can hook right up to a laptop though, I haven't looked around. I would recommend at least something using a USB connection, as most headphone jacks probably won't support the sound quality you want for monitoring. And I suspect that anything using Bluetooth would not work for recording, as there's often a time lag on them, and again, not full frequency response.
I'll save you the rest of my "home studio" rant, which I give often around here, but suffice to say it will help you make improvements in leaps and bounds as a player, and be useful for songwriting and demos, and who knows, maybe someday finished recordings. Of course, if the budget for that is a little much for you right now, some laptop speakers to practice with are still a great tool. But if you can swing it, the interface and monitors will do that and more. If you snag some on sale or used gear, you could easily get a basic rig for under $500. Just depends on your budget.