Wait are you not sending dm to new followers on slaves asking them to follow on your main account? That’s very important, that’s how we get 95% followers on mother account if you need help with anything you can message me, i have a setup of thousand of slaves providing manual m/s service
Will vary from different setups. If a slave perfectly matches a great mother account can be like 50%. If the slave or mother account is bad then can be less than 5%. Also sources of course are a big contributor to great folllow to mother.
IG still filters the messages, especially if the messages contain links. There are also cases wherein the users report the messages as spam and that may be where IG starts to look at those specific messages.
And yes, spintax is highly recommended, you may check out the link provided by @heroeslair