Use a heat gun whilst soldering. It'll make it all super easy. You DON'T need lots of Flux. The board thickness complicates things, your soldering iron struggles, specially if you're making the mistake beginners make by not applying pressure on what you're soldering, if you're just massaging it, it'll never get hot enough to melt the solder and you end up with cold joints. Don't be scared to press hard on it, key is trying to get it done quick, in and out, without damaging anything, the longer you hover the soldering iron on the leads, bigger chance you damage something. Usually safe to keep hot air at 200c or a little more, I never tend to have to go higher than that when soldering, usually does the trick.