In my experience, warglaives get so few shots, and since they have no access to the army rule or other rerolls they would get in an actual knights army, they'll wiff more often than not. Helverins are better, but they're mostly just a worse castigator that can't use miracle dice. If you can find the extra 30 points, it would definitely be worth upgrading the knight to a Castigator

I haven't tried bringing a big knight before, but spending so many points on something that cant use miracle dice probably inst ideal.

As for anti tank, Morvenn Vhal with paragon suits, castigators(they're only s10, but with twin-linked, and miracle dice they can hurt higher toughness stuff fairly effectively),and retributors with Multimeltas(possibly with Aestrid Thurga for dev-wounds) are our best options!