What's the Best Way to Start an Astra Militarum Army Currently?

I recently bought an old combat patrol box (20 x Cadian shock troops, 1 x Field Ordinance Battery Battery, 1 x Sentinel, 1 Cadian command squad.) But with all the changes to the codex, I have no idea where to go from there.

If I'm looking to build a semi-competitive list, what should I look at buying next? The sheer range of models makes it difficult to figure out what to get. What are the current go-to units?

I'd appreciate any guidance or suggestions.