... keep catapults the same but give them 3 range. Probably the easiest change.
Make Catapults the longest ranged unit in the first half of the game and I guarantee you Every Gamer will build armies of Catapults 'supporting' Melee or Anti-Cavalry Units, and never build another Archer again.
This would skew the game, just as the OP Battering Ram does now. Because its effects cover the entire perimeter of a city, you can conquer a Civilization one city at a time with just one Battering Ram, and it stays effective from Ancient to Medieval Eras. Not good design.
Instead, give Catapults a modest factor against troops, which makes them a little more flexible than the Battering Rams that precede them, make the Battering Ram effect only
the unit they are stacked with, and then give the Military Engineer the additional effect of giving a unit it is stacked with the Battering Ram ability to 'by-pass' walls (in this case, by tunneling under them!) BUT a Military Engineer stacked inside the city negates the attacking Military Engineer (Counter-Mining!).
Then the Bombard should, like the Battering Ram, have virtually no Ranged Effect on troops (they fired, on a Good Day, maybe once every few hours - any effect on troops was entirely accidental, usually by blowing up in the middle of their own troops) BUT Bombard's factors against Walls should make all Ancient and Medieval Ways effectively worthless - a very good reason to actually build Renaissance Walls, which almost no one does now.
Finally, when we get to Artillery, it should be the first 'siege' unit that also is extremely effective against Troop Units: artillery from 1914 to the present day far exceeds all other combat arms in its ability to cause casualties, despite all the publicity given to air power and tanks. (Full Disclosure: I was an artilleryman in the US Army for almost 20 years, but that statement reflects the findings of both the German and Soviet staffs studying WWI and WWII, and the French, British and US Army staffs who studied WWI)
These changes would, I think, put siege units in their place as specialized units required for taking fortified cities, and requiring careful handling to accomplish that goal. Right now, a Battering Ram or two is pretty much all you need to take any city from Ancient to early Renaissance, and all the other 'siege' units before Bombards are 'nice to have' but not really necessary. That is a waste of the programmers' and artists' time spent on those units, and makes the game play in regards to city attack that much more one dimensional.