As to the land/sea/air balance, perhaps you could do a paper-scissors-rock thing with safety in enemy territory. Normally I despise paper-scissors rock in games, but it sort of makes sense...
You could have land forces on an island hold the airfields and provide uncontested fire support to aircraft in their airspace, but take massive losses if they try to land in territory controlled by a hostile navy and can be starved over time.
Aircraft could harass naval operations such that navies in enemy territory would suffer constant attrition. Navies would be able to support land invasions but would be hard-pressed to engage the middle of an island where the airfields are, so would need to wait around for a land invasion if they wanted to keep going. Aircraft carriers would bend this a little, but they would consistently be constrained by a small number of small runways...
So land would sort of beat air which would sort of beat naval which would sort of beat land and it would be very expensive or slow to advance if any one of them were resisting effectively. Mostly this would involve making the islands big enough that a battleship couldn't effectively shoot the middle...
If you give players access to uranium, they will
want try to
use it...
Brace for players to try to change their affiliations half-way through the war or for both nations to want to join the same alliance.
I had an idea of splitting technologies into technologies, parts, and products. An artillery piece might use the 150mm shrapnel ammunition and the compact mechanical aiming system and the airburst fuses, which in turn need large-calibre munitions and compound munitions and mechanical turrets and altitude detection... you could, umm, keep them separate to limit the rate of progress in any one field or something. It sounds complicated now and my mind turned off suddenly.