Popular opinion: Ptolemies is the best civ for booming(getting a lot of resources and units) because of the team bonus for +1 food per second.
Another opinion: I think Kushites is the best civ for booming because of the pyramid aura boosting 15% gather rate. I can get even more food per second with a pyramid compared to the +1 food per second of Ptolemies, lets see how: If we put a pyramid adjacent to the civic centre and put farms around the civic centre close to the pyramid, we can get the pyramid aura boost of 15% gather rate on minimum 20 farmers. I assume 1 farmer gathers 4 food every 10 seconds without the pyramid boost. So 1 farmer gathers 0.4 food per second. 20 farmers gather 8 food per second. 15% of 8 is 1.2 so we get +1.2 food per second with the pyramid boost which is better than the team bonus of +1 food per second of Ptolemies.
Keep in mind I took conservative values, the food per second could reach upto 3 if we use the pyramid aura on 50 farmers. Bonus: if you keep the pyramid near the stone and metal mine near civ center you can boost that as well. + you can keep 1 pyramid on ur woodline. A problem is that kushites is an archer civ, so you will have to snipe to survive an endgame fight with javs or slingers. Sniping means selecting your archers to attack the enemy's ranged units instead of the default attacking the closer melee soldiers.
Elephants have better stats than melee soldiers (except speed) + they can break buildings faster so they're better. What you said is right, that's a risk if you're willing to take it, you can use the temple healing aura to heal elephants. You can garrison elephants in elephant stables. Elephants will die so you will have to replace them.
edit: If you're the moral type of player who wants to save all your units: We should be thankful we don't brutalize and torture elephants for war anymore, unfortunately its still done in most tourist spaces but on a lesser scale at least. Also any war back in ancient history would result in a lot of deaths, thats why people say war is bad. You can't escape from that in 0ad, in multiplayer people don't use healers and only focus on units who can do damage or eco cause thats what helps them win. Unless you maybe get like 8 healers to heal 1 elephant hero and make it invincible but non ai people would start killing the healers and then the hero
Yeah they weren't popular but Valhirant just made a video using kushites yesterday, I've also seen a lot more people play kushites in the multiplayer lobby recently.