This is insanely profitable! Honestly, usually such sweet deals are offered by MVNOs -- meaning not major "real" mobile providers (MNOs) that actually own the towers, but small secondary operators. These MVNOs rent part of the tower capacity and part of that tower IP pool from MNOs. And this pool will always be heavily limited! This could be related to your rotation problems. Plus these plans often have actual speed restrictions.
We usually don't recommend our users deal with MVNOs, especially if phones for proxy farming will be massively located in one spot.
That's an excellent idea -- if you can find a couple of locations (like at friends' houses) near the tower, then even the MVNO option might be acceptable.
I noticed you mentioned not seeing economic feasibility in selling proxies since your income mainly comes from other non-proxy phones. But just in case, I'll emphasize that if you do decide to sell excess proxies, it'll be important to store all IP history and logs in general for longer than just a few days. Sorry if this is obvious, but I often see people who don't pay enough attention to their security when starting this activity, so I always try to mention it. Especially since you seem like someone who doesn't use ready-made solutions/services (where the service handles logging for you) but builds their.
