In addition to Cyrus my other mainstay was Avitus. Once you get used to having Avitus laying down suppressing fire it is very hard to play without it. I rotated Tarkus and Thule depending if I wanted more shooty (Tarkus’ flamer is good for 'nids) or more biffo (good for taking out wimpy elves). Thaddeus is useless so I never used him.
The game does have an interesting meta decision to make whether to do the venom questline first (and get the dreadnought) or the angel forge questline first (and get the terminator armour). I think it’s good to get Thule first and start leveling him up, but I can see the merit in putting Tarkus in terminator armour if he is your main tank (which he is if you sink points into his green upgrade tree).
The only advice I would give on builds is focus on one upgrade tree for each squad, and junk every piece of equipment you outlevel asap for the extra XP.
Anyway it took me about 30 hours to play through DoW2 + CR so looking forward to the next 100 or so episodes!