Light Cannons are one of if not the best artillery unit in the game.
Regarding FAs, short version is one must look at Flaming Arrows (which aren’t amazing) in the context of the rest of the civ which has great infantry, great melee cavalry, good ranged cavalry, great wonders, good-great mortars, and the ability to train units on the front lines.
There is a very small buff that would likely be good, which is covered in here. The suggestion in relation to Flaming Arrows was essentially moving the +2 range from Exalted (Imperial) to Honored (Guard) so they reach 30 range in age 4 instead of 5. It’s a fairly small buff, but all that’s needed imo.