In my opinion a readiness mechanic would serve the game some considerations to be honest.
True and agreed
I would increase the cost for actively maintain ships and then introduce a reserve status, not the same as mothballing a ship.
Correct, I find myself in agreement with Aurora mechanics as I don't like the mothballing concept. It allows you to overstretch your military power and allows you to replace ships on the go once they are lost without any penalty.
The downside of "reserve" status would be that the ship loose half of its accumulated crew and fleet training instantly and then 5% for each year it remains in the reserves, this represent that when the ship is reactivated it will have replaced allot of the crew.
I think that this is too harsh. A fleet in reserve should not be able to accumulate any crew experience and should lose its experience to a fixed amount altogether. That amount could be the average between the existing training grade and the standard crew grading to refill the crew. So if the crew is 100% and it is decided that half get lost in reserve mode when you will reactive the ship you'll get half crew at (depending on your level and commander bonuses) let's say 17%. You can do the math but this should result in roughly 58%.
When the ships are brought back into service it should then reactivate in the same manner as a ship undergoing overhaul, it also have lost half or more of its crew and fleet training. A ship in reserve only pay 25% of the MSP cost for the maintenance clock to be paused if at a maintenance facility or the clock runs at 25% of the pace if not at a location with enough maintenance capacity.
It is reasonable to think that a ship should take time to be reactivated.
MSP cost should then be increased by twice the cost as active ships are more expensive.
I think the above it's too arbitrary and partially untrue. I would actually work on the impact on the maintenance tonnage of the maintenance facilities. So if you have 50,000 tons you can serve 50,000 tons of ready ships or a certain amount of reserve ships + active ships. Eventually, each ship should have a reserve tonnage amount for maintenance purposes. For example, a 5,000 tons ship could count for 5,750 for tonnage maintenance while in reserve or any other amount that could be "fair". This should also result in more MSP usage without necessarily being utterly costly and will put enough pressure on maintenance locations and infrastructures. In terms of realism and balance, a ship or a plane costs way more while not running as it requires engine tests daily, oiling of components, engineering calibration, and more. We saw it with COVID and the number of Sailing ships being scrapped along with planes as they haven't been used for too long and they cost too much to maintain. The active ships have different costs as they consume fuel, ammunition, and manpower; they also may break and need repairs. these concepts are already in Aurora so the costs of an active ship compared to the ones of a reserve ship are already in for me.
This would force some choices on the players if they want their ships ready and with high experience and fleet training or save cost on putting their ships in the reserve for the next possible conflict. This would be a pretty realistic choice to make.
I agree that the readiness of the fleet should be a factor with more aspects that build and wait for training to go up.