The Crimson Plague opens with a virus outbreak, so Virus Resistance is your very first priority - it gates whether you can fight infected zombies and the giant scorpions at all, regardless of raw Power.
Start Protein Farm and Virus Research Institute upgrades immediately - both feed Virus Resistance directly, and their costs only grow from here.
Pick a Profession as soon as it unlocks: Engineer if you're economy/building-speed focused, Warlord or War Leader if you're combat focused. Each has its own skill tree, so talk to your alliance before committing if you're not sure which fits your role.
If you have a 5-star Mason, this season lets you promote it to UR - a genuinely efficient way to turn a strong SSR into a UR without heavy spending. Worth checking your roster for candidates in week one.