I see everyone podding in their leviathans though. If it's on the board are you thinking of the storm cannon and grav flux? I think 18" is the longest ranged weapon on it?
Anywho, 30k is for sure rule of cool so if you dig it, go for it! 20 missiles is a lot. I think if you throw a siege breaker with 1 of the units you'll get good work out of them. I have run 5 before and it seems lackluster. 10 should do much better however.
I think some medusas or quad mortars with shatter shells will do well for your anti-tank. Laser destroyer vinidicators are also something I am intrigued by. Something that is a tad more durable would be a sicaran venator. Little heartier armor on that bad boy.
My 2 cents as a Raven Guard player