It's really just one blended strategy. You're already doing half of it. The other half is to use those 70 accts to build authority for the primary. A mix is probably best so it's not blindingly obvious that they're just slave accts. They can be rebuilt, but it slows you down and is far better to keep them going, so they can sometimes tweet, and sometimes retweet and this helps keep them from looking like obvious slave accts.
4k/day is good for 70 accts if it's well targeted, btw. It's possible to get more but the targeting is more important than the number itself. I'm being cautious to avoid bans, and laser focused in my targeting, and pulling a little 2k/day with 50 accts... but I know those are some of the best 2k folks I could possibly pick that are active tweeters and retweeters for my competition.
It seems to be the unfollowing that's the stickiest bit with TW and bans. What sort of rate are you running for unfollows?