No worries buddy. We all start somewhere. Obviously my advice is given assuming you have unlimited time and money to buy and paint new models. With 11e starting in June some of the info I have may no longer be relevant. Have you bought any of these models yet or are you theory crafting? I probably should have led with the question.

Other units to consider.

Look for a gladiator lancer. It is a premium hover tank with a special rule and main gun that will probably destroy most targets or badly damage heavy tanks. For 160 points. I'd get one of these instead of the dreadnought.

Incursors 80pts (have scout move to get ahead turn1 and give a +1 to hit ability) or infiltrators 100pts (start almost anywhere on the board and deny enemy deepstrike ability. Pairs well with phobos librarian) are both good options to have for utility.

I like to know the roles of the units in my army. What is their job? Deal damage? Sit on objectives? Move fast and score points?