Well when it comes to valuating an IG account, the three biggest factors are the niche, demographics of the followers, and engagement rates. You mentioned most of the followers are female from when the account was previously in the boy niche.
I would start by making a list of niches that you think a female audience would be interested in. Dresses, makeup, quotes, etc... your focus should be on appealing to the followers you already have and posting content that is relevant to their interests.
I would unfollow everyone you currently follow (don't do this too aggressively only unfollow about 100 people per hour). Then start grinding the follow/unfollow method day in and day out. Whatever niche you choose to go with, make sure to post niche related content 2-3 every day using relevant hashtags. I use Buffer to schedule posts and send them out automatically at set times for all my accounts, and they have a really good free plan you could check out. Makes life a lot easier especially when you manage multiple accounts.
Another strategy that works well:
Find trending posts in your niche that are getting a lot of likes and comments. Open the list of comments and "love" each comment. This sends a notification to whoever left the comment and will often lead to you getting a few follows.
Hope that was helpful, let me know if you have any other questions. Good luck!