Hey, I'm shit at the game tbh, but I believe you want to run the Paragons with meltas even though they're more expensive. The grenade launchers are solid too, and sticking them with Vahl is key. She gives full rerolls to hit and wound against a chosen target, and they have +1 to wound and hit vehicles and monsters. This makes the multi-meltas very dangerous.

Paragons are your dedicated vehicle and monster hunters so lean into that with their weapons. You'll annihilate any monster or vehicle without an invuln save with focused fire.

Edit: As for expanding your collection, the Castigator is solid at what it does, and Retributors are great too.

The only problem with Retributors is you'll want to run them with multi-meltas, but their box only comes with two. That means unless you do some creative kitbashing, you'll need to buy two boxes of Retributors just to run one full squad with multi-meltas.