Transfer of apps, and app data between Apple accounts.

TLDR: I got a new phone, and with it, my own Apple account. I denied the transfer of apps, and app data to the new phone from the old, because the old is a tangled mess of a family account (of 4)s' apps, subscriptions, passwords, and contacts from at least 15 years back. I've solved some things regarding contacts, and have some idea of sorting out passwords. I've lost my subscriptions, since I have no money for them, and have found the solution to the TestFlight worry I initially had for a closed beta. So, this is why I am coming here, for the last part of my worry, paired with a couple of questions. Has anybody else had this kind of experience, when branching out into independence, where Apple accounts are concerned? If so, and even if not, do you have any tips for making sure I have thoroughly found everything I would like to keep before trading the phone back? Also, what of the payed-for apps, and purchases? I have a game (I believe costs money), and own it on the old phone, but since it is still me, but my own account, is there a way I can get a code, or something, so I can just keep the app, on the new phone. Other games (Perceptron, Cmd C, and a few others, I am less worried about, because I can just return to the place I am at, when I play the game.) My biggest worry is the in-app purchases. I have the DLC for Dawncaster, and the purchaseable stuff for Home Quest, for example, that I'd like to be able to restore, on the new phone. Sorry for the long-windedness of this, I am just trying my best to get everything before I send the phone back in.