I plan to make a second main account, to work with my main account, which is already at 15k followers.
I will of course put it into MP, and try to get it’s own 1:1 4G proxy.
Is it a bad idea to set it up on the app on my phone, so that if it gets flagged it may affect my more important 15k account?
No, the only account get banned is the one abused instagram policies. Your 15k account wont get effected.
Zack is 100% correct here, even though it may seem counter intuitive. I was told by companies like Instavast back in the day that having similar profile images and names could affect the trust rating of a main account. This seems to be logical at first, however take into consideration that there are sometimes hundreds of fan accounts for celebrities spamming themselves to death. If similar accounts affected main account trust score then you could easily destroy the trust score of any account.
This being said, perhaps it would be advisable not to run both accounts on the same device given their similarity. I’ve experienced shadowbans in the past doing this, even though I ran them on separate IPs.