Seroquel, other than being an antipsychotic/mood stabiliser is a potent antihistamine, similar to certain types of drowsy allergy medication and over-the-counter sleep aid and cough remedies. For this reason, sedation and appetite increase is a common side effect.
That being said, it is nowhere near as bad as weight gain on Olanzapine or Clozapine. From what I hear, the drowsiness does tend to pass after a month as your body becomes more tolerant of the antihistamine, but not sure about the weight gain. I would say give it a go, at least for a couple of months, but try and stick to a healthy diet and a regiment of exercise, otherwise I doubt you'll be pleased with the result. As a person who has struggled with weight (not medication related) all my life, I cannot stress enough how important these two are. I would recommend trying a time-restricted eating diet such as intermittant fasting or OMAD/TMAD. Its harder to disciple yourself in it, but easier to follow and keep track of how much you are eating, as oppose to calorie-control based diets which require you to measure out each meal and figure out how many calories you are eating. In terms of exercise, do cardio, especially interval training and lower body strength training e.g. squats.
It might seem like a lot of effort, just to ward off weight gain from a drug, but I recommend strongly. Firstly, because its good for you anyways. Second, because Seroquel is actually a very good drug for bipolar. It wards off both depressive and manic states in bipolar, and can aid sleep which is essential for preventing mania. However, if all else fail, there are alternatives to Seroquel (Quetiapine). Abilify (Aripiprazole) (which I am on) and Amisulpride tend to be weight neutral and sedation free, however only prevent mania. But since you are on Lamictal, that ought to prevent the depression anyways, so it wouldn't be too much skin off your nose to switch.
I will list below a comparison chart of antipsychotics and their side effects.
