A lot of people use www.scrollbuilder.com to build lists now. Most comp systems seem to base things around grand alliances, but I don't think anyone minds taking units from wherever you want.
The game is definitely strategic and tactical, possibly in some different ways to the old rank and file system. There's a lot to think about around synergies, setting up for a double turn (or setting up to be able to withstand a double turn), order of combats and I'd say especially using the fleeing mechanic.
Everything good about the game comes out best in scenario play, which is what
AoS was meant for. Battle line can be okay but is bland and can turn into a grind.
I'd say again scenarios are necessary and I'd recommend starting small and building. Trying to put in 100+ models a side like an old fantasy game and doing battle line in your first games could be a slog.
There are still lingering ambiguities in the rules but the spirit of
AoS is about agreeing with your opponent about how you want to play.
I'd recommend checking out Heelan Hammer and Facehammer podcasts. They are both event organizers and attend a lot of the
UK scene's
AoS events and talk a lot about "competitive"
AoS.
Wasn't a fan of Miniwargaming's early
AoS videos as all they ever did was battle line and it was boring, but they've started doing scenarios now and I think it shows off the game much better. Here's their last one:
https://www.youtube.com/watch?v=-FOUA0-amks
Also recommend Guerrilla Miniature Games on Youtube.