Can Rhonas the Indomitable attack if I pump Voltaic Brawler?
You can't get Rhonas attacking that way, unfortunately. This is because of how we handle declaring attackers:


We enter the Declare Attackers step.
You choose attackers. They're declared attackers simultaneously. (At this point, Voltaic Brawler can attack, but Rhonas cannot.)
The active player receives priority, and in the process of them receiving it, triggered abilities go on the stack, including Voltaic Brawler's.
You get the opportunity to choose to pump Voltaic Brawler with +1/+1.


At this point, declaring attackers is done. You can't declare new ones. It's too late.

This would be different if you used Rhonas's activated ability before combat to get a 5/2 Voltaic Brawler before declaring attackers. (That's exactly what it's there for: to benefit your strategy and help Rhonas attack. Every Amonkhet god has an activated ability that moves you closer to them being able to attack and block.)