Tyranids, my venture into painting

So, this is my first venture into 40k. My first painted piece, actually. (The photo is kind of poor quality because I took it with my phone). Any advice for someone in the beginning stages of painting an army? Do you take a great deal of time on each piece or concentrate on the bigger models? And I've heard about using the magnets for larger pieces, do you think it would be worth it to also use them on smaller pieces?
((Sorry if I'm asking too many questions, but there aren't a whole lot of people in my area that actually play/paint. Thank you in advance for any advice you can give!))
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Looks super. Make sure you can replicate the effects easily on a whole army, though.

The way I determine what gets magnetized is the cost/time of doing so vs. the cost of just buying a second model. Eldar Guardian with flamer or melta? Make one of each (though really, more like, why bother with the flamer). Wave serpent with TL whatever = magnets.

That said, I just bought a ton of 2mm magnets to swap out hand weapons vs. spears on a bunch of savage orks, but that's like 30 dudes, not 2. Plus I just got a low rpm dremel which I hope will make that process faster (or just destroy everything more quickly).

always magnetize bases, makes it so much easier to transport

Take your time on each of your miniatures, but break the time up into phases of painting. Start with your basic 3 color scheme for all the minis you'll field, then work your way around to additional hues of each color, detail your favorite characters, spend time on making them your favorite miniatures. You want to get your force playable, then beautiful. But don't rush to the final phase of painting for a tournament or a skirmish, they're'll always be another scrap and a chance to dazzle your opponents with your finished paint scheme later, but you can't strip away a rushed paint job as easily.