Welcome to the site, sprout.
![Smile :)](./images/smilies/icon_e_smile.gif)
I just got here myself.
I think your plan is solid, and it's very similar to what I'm going to do. Your hlg-600h-42ab driver has two outputs, so I'd build 2 fixtures with 10 strips each. Parallel wiring sounds right afaik, but I'm not an electrician, and the other strips on angle aluminum should take the heat in the event that 1 strip fails. Also, I think there are methods to use the driver's pots to match/limit the voltage to the strips, but you'd have to search around for it.
My only concern about your build is that you want to use thermal grease between the strips and L angle. I've used a ton of Arctic Silver over the years in PC builds. But for strips it doesn't make sense.
Why? Well,
1) it's totally unnecessary because these strips don't have as much concentrated heat output as computer chips, so while you are increasing the dissipation capacity, the strips don't need it and don't benefit that much from it;
2) it's really messy and I can't imagine how many weeks or months it would take to apply that much grease to the strips;
3) if you ever had to take the fixture apart, it would be a real pain;
4) the double sided thermal tape normally used in strip builds + a couple nuts/bolts/plastic washers is plenty to attach the strips to your L angle, and plenty to manage the heat they put out.
You will definitely stress the plants if you don't dim the light and hang high at first. Luckily, you have the ability to dim on the driver or connect a dimming potentiometer to the proper lead from the driver. Get a Kill-A-Watt or similar and dim to half of max wattage for the first week or so, and hang 24" above the plants while doing so. Over the course of a week or two, slowly work your way down in hanging height and up in wattage until your plants show signs of stress, then back it off.
In a 4x4, your build is going to be amazing. I look forward to seeing the finished product!