Witchlings are absolutely essential for Sonnia, because you can't summon a model you don't have. Now personally, I like to bring an Austringer (good minions in their own right) and three or four Raptors, because the Raptors are the best targets in the world for getting blasts off Flame Burst, being highly maneuverable and Easy to Wound. They can also be good fodder for Violation of Magic, because the moderate on Flame Burst takes them to one wound. Witchlings themselves are also great fodder for Violation, because the severe on Flame Burst takes them to one wound.
For objective grabbing, I like Watchers. They're cheap, significant, and move seven. I'll keep a couple Watchers in my deployment zone for the first half of the game, using their valuable ability to look at fate cards, and then I'll send them out to do stuff. They can be a little squishy, but at least they're immune to disengaging.
The Brutal Effigy has become an auto-include for me, even though it prevents taking a totem, because healing. (Also, if the plan is to manifest, the totem would go away and the Effigy does not.) Effigy is also good in combat, but mostly I love the healing, because there isn't much of that in the Guild.
The Guardian has been mentioned, and Guardians are good, but Sonnia doesn't really need one. Her sword gives her healing when she hits something (btw, doesn't say it has to be an enemy model) so she's pretty survivable. She can also block scary things a little bit with the Flame Wall, and can be essentially immune to spells since she has counterspell. However, if you want to get Santiago Ortega, it's a good idea to attach a Guardian to him, because he's got a sweet spot of wounds where he's a brutal killing machine. That's a combo for high-stone scraps, though, because they both cost seven.
Always always always make sure Sonnia has a full cache of eight stones, because if you have to Confiscate Lore to make all your casts work (which you will) it takes a stone, and there are games where she will be eating stones like chocolates. I've actually thought about getting some Ronin for her, so she has a way to get more stones if necessary, without having to resort to Drain Souls. (Ronin also ignore armor, so I'd like to take them against Ramos, and they're Hard to Kill, so perfect Violation fodder if that's needed. Man, I really need to get some Ronin.)
Sonnia can hire Fire Gamin, so if they're something you already have as an Arcanist player that's another option to consider. I haven't found them to be super useful yet, but if you can keep them from dying, burning counters can definitely be a deal for avatar Sonnia, once you get to that point.
Sonnia is probably my favorite master, so I think you've made a good choice, here. Good luck!