The First Order has more than 30 ships. Snoke is riding around in that 60km monstrosity that doubles as a ship yard. They are also a galactic level threat, 30 ships, Star Destroyers or not, are not going to conquer an entire galaxy.
Also, blowing up a few planets isn't all that wasteful if you're considering things on a galactic scale. Honestly, if you're talking about something like mining or ores, all you've done is turn mining a planet into mining an asteroid field. Depending on the planet, that might actually be easier.
Unless a planet has a unique resource that is irreplaceable and completely necessary, blowing one up isn't going to do that much harm to an economy. If that was the case, it's an all eggs in one basket scenario and a solution would be needed to fix that situation.
It would also have been very effective at killing off direct resistance from any rebellion. The rebellion still would have existed after Yavin, it just would have gone underground and become more radical. They would have really become terrorists.
The Emperor was never planning on going on a planetary killing spree. Yavin 4 probably would have been it. Other than that it would have been put into orbit around planets on occasion as a threat and likely never would have seen any more use.
It was pretty much just a big stick, and I doubt it would have been used unless the resource cost of dealing with an uprising or rebellion on a planet was higher than the cost of just obliterating it would have been. There still would have been Star Destroyers and that big military machine the Emperor had built.
Also, he wasn't putting all his eggs in a single basket. There was always going to be more than one Death Star. Probably several. Construction on the second was already started when the first one was destroyed as I recall. They weren't just super lasers, but were also mobile space stations and military bases. There was probably going to be a network of them with one assigned in a sector or region of the galaxy. That makes a lot more sense from a military and strategic standpoint.
The were never going to replace Star Destroyers or anything else, but rather were going to be super space fortresses that the fleet would operate from and would likely rarely ever see any action, even though they would all have that big super laser built in as a deterrent.
We also know that it didn't have to destroy an entire planet every time it was used. Jedah was a pretty effective demonstration of that. It also functions as a tactical nuke that can take out a city without destroying the entire planet. Though, life on said planet isn't exactly going to be nearly as nice as it was before that happened due to fallout. It has been shown that it could be used to deal with military targets on a planet without wiping out resources. Though again, that depends on what the resources are exactly.
Also, Jedah was a moon and not a very large one at that. So the negative impact shown in some of the comics [which are canon] would not be as bad on a full scale planet. Even then, there were survivors several years later.