In my opinion, if you managed to register 8 accounts on the same device with the proxies you're using, you’ve already won half the battle. Once you’re on an iPhone, proxy quality becomes the most important factor. I’ve seen many cases where people couldn’t even create accounts just because their proxies weren’t good enough. So it looks like you’ve found a solid proxy source, which is probably why registering all 8 accounts on one phone worked out.
Once the accounts are created, just rotate them in shifts for warm-up. You’ll still need to attach the proxy to each account before warming up, right? Things like browsing the feed, watching Reels and Stories, doing keyword searches, repeat that for 2–3 hours per day per account.
That said, if you have the resources, I’d still recommend using an antidetect browser like Hidemyacc, Dolphin, Gologin,etc. to log in and post once the accounts are created. It saves you time and effort compared to manually switching proxies and accounts on the mobile app every time.