Originally I had a local electric company take over our security system's sensors - it seems like the sensors are mostly standardized hardware where the sensors just read open or closed to some central hardware box, and the actual monitoring is handled by some third party. Might be worth investigating.It would have been a factor of ~1k more expensive to go with ADT for us.
Ring Alarm does offer a "wired alarm takeover kit" apparently: https://shop.ring.com/products/retrofit-alarm-kit