Constipation and general diet recomendations

I am new to eating vegan, made the transition about 2 months ago, but I have been eating very close to vegan (some fish/dairy/egg maybe 1-2x a week) for over a year now. Since I transitioned to this diet I have been having bad constipation (a BM maybe every 2-3 days and output is much less than input). Up until a month ago I was going 2-3 times a day.
I have to take Miralax on a daily basis to keep the production line moving along otherwise I am having serious stomach cramps. And taking Miralax doesn’t even make me regular on a daily basis.

I think the underlying issue maybe that I don’t eat a diverse diet and probably not enough calories.
On a normal day I eat a 1/2 cup(before cooked) of oatmeal with some berries or walnuts for breakfast, for lunch I eat half a cup of rice(cooked) with some beans and for dinner I eat a chopped salad that consist of 1/2 white onion, green pepper, tomato, radishes, mushrooms, 1/2 avocado and a 9 oz. bag of romaine lettuce (no dressing or croutons). In between I usually eat raw veggies or apples & oranges when I feel hungry. I drink plenty of water (I am guessing at least 60-80 ounces a day)

I read that lettuce can cause constipation but I have been eating lettuce 2-3 times a week on a regular basis for years, although since the switch it’s more like 5-6 days a week. Is that too often to eat lettuce?

Maybe I am not eating enough calories and that’s why I am constipated. I read somewhere that malnutrition can cause constipation. My basal metabolic rate is around 1,450 calories when I last had it tested and I usually work out 4-5 times a week for an hour.

I try to eat raw veggies as much as possible, I also find it difficult to eat processed foods any more. I struggle and have to force myself to eat pasta, bread and even potatoes.

Am I eating enough? What should be my calorie target? (My BMI is 23.7 and I am losing maybe a pound a week)
How can I stop the weight loss without stopping exercising? Eating all these raw veggies makes me feel full and I am hardly ever hungry.

Can you suggest raw food items that I can add to my diet (and available in a regular grocery store)?