Wired Soil Moisture Sensor suggestions

I’ve read through a lot of threads and am not seeing something quite as specific so posting here in the hope of getting advice and helping someone else out in the future.

I have just “upgraded” from the old RainBird ESP-X-12 from 1995 to the Rachio 3 (12 zone) running iro3-firmware-hk-632. So far, so great!

The folks who installed our irrigation system 25+ years ago over-provisioned the cabling, leaving 2x wires running to each zone’s irrigation box. This got me thinking that it would be awesome to buy one or two (presumably inexpensive) wired moisture sensors to help Rachio be even smarter about when to water.

However, I’ve been surprised to find that the options are pretty “hard core” and difficult given my setup is all built into walls and conduit under paths:

  1. There’s a Rainbird option for $150 which has its own heavy duty cabling and which works by completely intercepting the common circuit. I get why this is smart for a “dumb” system, but aside from the cabling PITA, the idea of “jamming” Rachio this way seems the opposite of smart. The fact there’s 4x wires also knocks it out of contention for my situation.
  2. There’s a TORO option which is even more expensive than the Rachio and feels like overkill being wireless and all.

Has anyone else found a solution which inputs into the “Sensor” circuits on the Rachio and which is powered using the common + one other wire to provide inputs Rachio can use to determine whether it is time to water again? I’m not that experienced with electrical engineering, but I would have thought something which uses conductivity of soil using a pretty low voltage and provides a thermocouple-style reading which Rachio can use should be pretty simple (and not require a three figure spend).

Any tips from more experienced players at the smart irrigation game?