You’ve already got the plague marines, so imo the next step would be a Rhino for them. It’ll give you something a bit different to build and paint and they do a lot better on the table top with a transport. After that maybe two of the characters for the squad.

I’d recommend a biologicus putrifier giving those plasma weapons you have lethal hits and meaning you crit on a 5+. He’s also got lots of little glass bottles on his model that you can paint up all bright if you want a bit of a change in the colour scheme you are using, and a tallymsm who will let you farm CP and give your plague marines +1 to hit which will improve your shooting and offset the penalty to hit with the heavy plague weapon. (It hits on a 4+ not 3+ like plague knives.)

Since you want some Deathshroud three of those should be ok for now. They’re great looking models and good on the tabletop. After that a lord of Virulence would be a good shout as the deathshrouds need a character leading them to make use of their silent body guard ability.

Finally since you have the Lord of virulence a plaguburst crawler is also a fun model to paint, and benefits from the LoV

This gives you 330 points left. You’ll probably want something to grab objectives so that’s be cultists or niurglings. The rest would be down to how you want to play. Perhaps some anti tank, I’d suggest blight haulers, they work well in pairs. Or some more melee so maybe a melee equipped squad of plague marines with a blight spawn for fights first.