If we limit ourselves to only altering the top half, we rule out a lot of possibilities
You can still easily adjust unit hammer cost.
I just don't see any advantage from removing 1:1 resource requirements, which are a nice simple thing that is easy to understand, and are how vanilla works.
Given that its possible to achieve balance without changing this mechanic, I see no particular reason to do so.
What suggestions do you have to buff battleships to equal value as tanks?
I haven't tested your latest stat tweaks for them, but its perfectly possible to have a very durable long-range artillery platform that is incredibly useful (particularly for bombarding cities, out of range of almost everything), unless you're playing on a map where there is very little that is close to the coasts.
In which case, use aircraft instead.
Basically, we agree sea-based artillery shouldn't surpass their land-based counterparts, right? It wouldn't make much sense when looking at real life.
I agree that frigates shouldn't be better than cannon, and that destroyers shouldn't be better than artillery.
I don't agree that *battleships* shouldn't be more powerful than a single artillery unit.
Battleships should be a specialist, heavy naval unit.
Realism isn't a useful guide here, because:
a) We don't know how many ships/how many artillery pieces are represented by a single unit
b) The game isn't really capable of representing the real value of naval units because we don't have super-valuable naval trade routes, so we have to "fudge" a purpose for them by making them ground support troops.
c) We don't want "realistic" battleships, because in the real world battleships were nearly useless.
Since the value per turn of the value/cost equation can't be increased much,
Sure it can, for the specialist units.
And for aircraft, I imagine that a small strength increase could drastically reduce the amount of damage they take from non-AA specialist ground units. The amount of damage they take is my biggest problem with them.
I am slightly worried though that there is no way to make them take less damage without also making them inflict more; I worry that aerial bombardment combines unit strength and ranged attack damage in the same single parameter.
and I'd personally rather have the modern armor since it can capture cities
So what? So can mech-inf - and just as effectively, thanks to the city-attack penalty on modern armor.
If you're using tanks to attack cities, you aren't using them very effectively.
In fact, I'd much rather have a battleship as a support unit for taking cities (which can pound away without taking much damage in return) than a tank.
Specific numbers aren't important at this stage
How so? What you are doing is precisely tweaking specific number values.
How can you do balance without specific numbers?