Bombardment as currently proposed is useless. Killing 1-2 resource/civic improvements is very, very small. At 35 cities @ 3200 infra I'd go into a war with about 1640 resource/civic improvements after military/power. Even a 20 city nation @ 2500 infra will go into a war with about 600 resource/civic improvements. If each improvement is worth 10-20k in profitability per day (maybe up to 30k, but we're talking averages), loosing two improvements is about 1 mill in income over a month. It would take many months to pay for the gas/munitions even with the 50% reduction.
It should scale up with how many ships are used, which also serves as a good replacement for the "75% of max capacity" mechanic. 1 improvement for every 50 or 100 ships used. This has the added advantage of making it meaningful for the upper tiers without being overpowered in the lower tiers. The more cities you have, the more improvements you have, the less each improvement is worth to you, and the more the attack costs.