My suggestion is really simple but one that really needs doing .
You cant declare on people who are more than 5 cities below you(doesnt matter if you are within score range). But u can declare on people more than 5 cities above you. As this recent war has shown the big nations can easily drop some infra then declare on people 10+ cities below them then destroy them with there higher mil recruit count per day which makes it un fair on smaller nations
Nation A has 30 cities
Nation B has 20 cities
Nation C has 25 cities cities
Nation A cant declare on Nation B but can declare on Nation C
Nation B can declare on Nation A and Nation C
Nation C Can declare on both Nation A and nation B
This helps protect smaller nations from being ambushed unfairly by bigger nations who just sell infra to get in range. But it wont change the game that much or the war system as u can still declare on nations more than 5 cities above you.
This still keeps the score range for declarations just stops you from declaring on someone too many cities below you(even if in war range)