Chimney
Trakanon Raider
What's the best way to push way out and stay safe? Push a little. Build a wall and towers, push a little more, build more walls and towers?
While you are expanding you should use whatever army you have to continually push forward until you get to large populations of zombies or doomtowns. From there you can just patrol perpendicular to the swarm you need to clear and if you notice runners or too many mobs then kite back a bit until you can keep pushing. One ranger can clear a surprising amount solo and being solo it'll hit vet faster adding to its clear speed. Just pause periodically to work on economy while dealing with micro. Once you hit day 50+ you should be at the point where you have an army death ball (I tend to use 20-25 snipers and once they all hit vet they are pretty much invincible) clearing map while focusing purely on macro economy.
I personally don't build full walls until around day 80+ and that's usually just for the area that announces where the wave is coming. If you drop a ballista and put 2-3 posts in front of it that's enough to keep it safe. Same with teslas or whatever else you are pushing. The zombie AI will 99% of the time go right to the wall instead of whatever you've built allowing time to get some army to clear it if you don't have a ballista covering. I also can't overstate how kiting is super OP. You can split huge waves or kite them in circles while ballistas do the work with 1-2 rangers.
Pay attention to where swarms focus you'll notice that they tend to clump in one section leaving the majority of your wall/ballista defenses as wasted resources. It's not until that last wave where you really get them spreading out a bit.
Obviously it's all down to your preferences, but since swapping to this play style my success rate has gone up a ton compared to safe/slow expansion.