I've been working on a TEMU farm with a bunch of old phones (around 20) and the factory reset trick works great, actually.
My only problem with doing it is that most of my (very) old phones are so low quality and so old that they can barely even run chrome and some can't even run the play store to install TEMU. lol
But with phones that can handle it, the catch is, that TEMU sees your wifi and your IP, so it will start prompting you for phone numbers to confirm new accounts after the first few. wait a day or two and it will reset and you can create a few more without a phone number before being prompted again.
To get around this, get your phones set up and then go to different locations with free wifi or a friend's house and create accounts on multiple wifi signals. You can probably get away with 2 accounts per wifi without needing numbers.
You'll also need an email for each new account.
Gmail is good because it also gets you logged into the playstore to install, but Gmail requires phone number verification after several new accounts as well.
So if you're seriously getting into a TEMU account farm, you can just alternate days of creating gmail accounts, and creating Temu accounts. plan to use several locations for wifi to register both and you can get more accomplished.
So far there seems to be no issue with sending all accounts orders to the same physical address and the same phone number for the ORDERS seem to be okay being used on every account. (so far)
So as long as you can create a steady flow of new email accounts, new phone numbers for verification, and can alternate wifi periodically, you should be able to maintain a fairly steady TEMU farm factory resetting old phones.