I made a land transport unit that carries my artillery along just as fast as my tanks go. Drop the guns and let the preliminary bombardment begin!
For conventional rules, I still find waiting for my artillery to arrive is faster than replacing cav lost assaulting infantry/riflemen, or for those cav to heal up afterwards.
IMO, use of catapults or cannon is just a waste of time offensively, so I don't bother with bombardment before Replaceable Parts.
My typical bombardment invasion force consists of two infantry escorting about 10 artillery. I have never seen a computer dare to attack this force with ANYTHING, despite the fact that the arty's not fortified because it's on the move. Generally, one turn of bombardment with such a stack will reduce all defending units to one or two hitpoints, and at that time, I'll send in the cavalry.
If bombardment reduces the city population below 7, great. If your intention is to reduce the population, not beat down defending units, I would recommend pillaging the irrigation and just letting the city starve down to size rather than pounding the city itself. Obviously, you don't
need artillery to do this, but a stack of arty on a well-placed square can often lay waste to a good chunk of the irrigation to multiple enemy cities from a single location, without exposing units to counterattack by skirmish units in those cities. Often, this type of "scorched earth" fighting will drive an enemy to their knees and the peace talks, even if you don't actually take any cities.