Great core you have already there, and agree with the above mention of adding rider for even more mobility and killing ability.
Silirids i found to be worth their weight in gold and one of the factions best scheme runners.
I also used spawn mother quite a bit, summoning multiple gupps as a road block, surprise scheme runners or just a general PITA always seemed good value, especially as a focused charge from her is a painful experience for the opponent too.
Otherwise, don't be afraid to forget obeying when the time is right, 3 hex attacks at a range of 24 inches is enough to kill a large number of things, especially if they're engaged.
People don't rate him, but I found him decent (and also if you want that extra beater to be thematic instead of rider) then McTavish can do some damage too.
So post waffle, I'd say McTavish and spawn mother are the only 2 things I'd consider grabbing personally, but what you have already seems pretty flexible at most S&S