300 wouldn't be enough for me, specially in the second and third trimester and when nursing. I need atleast 500, but more often 600. If I were to limit food so much, guess where the nutrients for my baby would come from? They would need to be stolen from my own bones and my own body.
300 should be adequate for the first trimester, but your body is telling you it needs more calories. Or maybe... different kinds of calories.
The general advice is to increase the fiber to stay full longer, but that never worked for me. After 30 minutes, I was shaking and my vision would go blurry and black, and I was too weak to walk or push open a door.
The only thing that helped me was eating fat. I've been eating like that for more than 3 years and eating this way I only get hungry every 6 to 8 hours, and don't have symptoms of diabetes anymore. Even though I eat a lot more calories and fat than before, I lost a lot of weight (body fat).
Our bodies can't digest fiber, it's just a filler taking room in our stomach. You mentioned empty calories. The way I look at it, fiber is empty nutrition. So I chose to replace it with foods that keep me full and provide nutrients.
It's not weight gain that gives you diabetes, weight gain is a side effect of diabetes. You get fat because you keep burdening your body with high-glycemic foods to the point insulin can't do its job of reducing your blood sugar anymore.
Can you eat a big breakfast in the morning to start the day well, in terms of preventing hunger? I saw that you have chickens, so maybe two (or three) whole eggs cooked with one or two generous pats of butter, and vegetables on the side? Like, chopped cauliflower cooked with chicken broth, or sliced onions and red peppers cooked in butter.
See how long it keeps you full. How about a egg quiche with heavy cream and lots of spinach or swiss chard?