So I thought I messed up my calculations for a second then realized I had them technically right even though I originally misunderstood the weapons stats page which caused me to delete and now remake my original comment, messing up the formatting.
Not sure, just going off the data posted here. According to my calculations, the 12pdr Howitzer does worse damage than the 10pdr Ordinance at anything above ~225 range, and considerably worse at 300 to 600 range. And up close, the Napoleon does considerably more damage 300 and in. The 10pdr Parrott does comparatively good damage above 600, although not as good as the super limited supply Tredegars. However, at this price comparatively good damage means pretty limited damage at range. See below for 20pdr Parrot.
The Richmond seems pretty meh, cost-effective wise. Only slightly better than the Enfield at 200 range and worse from 250 to 300 range and similiar fire rates. Yes, it is better up close than the Enfield, but not significantly better than the Lorenz up close, which is only $22.
That being said, I have watched your CSA legendary playthrough and have no doubt you are a better player than me. Maybe I really am messing up my calculations or not understanding the data properly.
The 20pdr Parrott is worse than the 10pdr Ordinance up to 600 range and worse than the James from 600 to 1400 range. It seems the 20pdr Parrott does consistent but pretty negligible damage at all ranges. Considering its price and the fact that it has one of the lowest firerates in the game, it seems bad.
One other thing I forgot to mention, is that it looks like the type of shot (canister, shell, etc.) does not actually effect the damage and is an animation only. Only range affects base damage. Maybe the different shots have different aoes, Idk.
The above all just represents my personal opinion, biased by my playstyle for what ranges are most important, based solely on the data which OP very generously provided. I should also note that I am somewhat ignoring variance given the amount of volleysfired during a campaign. the 20pdr Parrott is much lower variance than all the 10pdrs.