[Alneyan]

Nobody said the Star Destroyers should move unescorted: you could move them after the bulk of your fleet, as a "no counterstrike measure", or you could simply move them if you have accurate intelligence on what lies on the other side. More importantly, you can use them on your borders (on in your own systems), where you have a strategic advantage: even if your foe does head for the star(s) in your systems, a Sentried Star Destroyer can still do some damage.

The main problem is putting the Destroyer in the same system as an enemy fleet; that one usually requires you to have either a Stellar Manipulation dominancy (unlikely), or to have a central "hub" of wormholes allowing entry into your systems (or System Gravitational Shields, but those prevent the Sentry Destroyer from working). Alternatively, simply put one such Destroyer in every single system, especially if the enemy likes to use big fleets together.

Ragnarok can outproduce the enemy here, so there is no worry with SYs. Still, a large Empire in the later stages of the game should have a few hundred of planets, many of them with asteroid stats, other breathers under direct control, 70% resource conversion, and a rising Monolith economy. This stage should be reached around turn 80, more or less, depending on your trading/treaties and the specific circumstances. In a 2vs2 high-tech game, I ended up having about 300,000 minerals to spare turn after turn around turn 70, despite having about 150 SYs and a few Star Destroyers under production, plus a small fleet of 350 to maintain; since it was a warring game, my Empire was definitively not optimised: no Monoliths, only one breathing type, no asteroid planets.

In another game, Joachim had about 20 million points by turn 120 (NeGoC5), roughly 1,200 colonies, take or add one hundred, and a very sturdy (and studly) economy. Their fleet was *big* and had the Talisman, and he could still go on with pumping more warships. When you have this kind of Empires, putting a mere 30 SYs to Emergency Build is no worry: you should have quite a lot of SYs already, including Base SYs if needed, and your fleet should be sizeable to say the least. If you have the resources to spare, you could also build the basic hulls of your warships in the SYs, and do the rest of the work by retrofitting. It is more expensive and time consuming in micromanaging, but *much* faster, and the only limiting factor is how many resources you have. If your SYs can crank one Dreadnought every two turns (then retrofitted in orbit for two turns, perhaps three), you should be able to build as many warships as you need, and then some more, so long as your economy can keep up. And if you really need more SYs, you can add some in your colonies: scrap a Research Centre on all your colonies without a SY, and build a SY there.