What can a large dose of D3 cause in terms of imbalances? I read it competes with Vitamin A, and theres also a vitamin K and Calcium link. So would a high dose vitamin D cause a deficiency of K and Calcium, or an elevation in calcium/K levels?

Taking excessive vitamin D can lead to abnormally high serum calcium levels.  This in turn can lead to high blood pressure, increased risk of asthma, muscle cramping/twitching, nausea, vomiting, depression, irritability, brain fog, etc.

Vitamin D also increases intestinal absorption of phosphorus leading to abnormally high levels of serum phosphorus.  This increases the risk of calcium phosphate stone formation, which most often affect the kidneys.

There should not be much of an issue with vitamin K as the vitamin D will be mostly absorbed long before reaching the colon where most of the body's vitamin K is produced by the flora.

There are various views on whether or not vitamin D is toxic in high levels and clinical cases to back both sides.  A lot of the confusion can be explained by various factors such as the form of vitamin being taken, if the vitamin D is produced by the skin or taken orally especially with calcium and/or phosphorus and the type of vitamin D being taken, how well it is being absorbed and the forms ingested and produced in the body.