Here is the best method I have come up with (it assumes your harvester is running on a track):

There are 2 variations on doing it, depending on if Supergridding is enabled or not, but the whole point is to let the harvester "see" the center point of the farm plot.

If Supergridding is enabled (best and easiest method):

On your main grid (that your farms are attached to)
1. Set up a line of rotors facing up or down at the same spacing as your farms.
2. Remove the heads.
3. Add small heads.
4. Lock the rotors.

On your harvester:
1. Set up a sensor facing upwards or downwards as appropriate, aligned with your Small Connectors that you're using as seed droppers.
On the sensor you just placed:
(a) Turn OFF Detect Player and turn ON Detect Small Ship.
(b) Set your ranges for Top / Bottom / Left / Right to 0.5m. Set Front / Rear as appropriate.
(c) Assign the Detect action to the Drop Item command on your group of dropper connectors.

Toggle the sensor on and off to enable / disable seed drop functionality.

If Supergridding is NOT enabled:

On your main grid (that your farms are attached to):
1. Set up a line of rotors facing up or down at the same spacing as your farms (this will most likely need to be about 3 blocks under the floor for reasons that will become apparent shortly). The direction the rotor head is facing must be towards your harvester.
2. Place a pillar on top the rotor head.
3. Lock the rotors.
On your harvester:
1. Set up a sensor facing upwards or downwards as appropriate, aligned with your Small Connectors that you're using as seed droppers.
On the sensor you just placed:
(a) Turn OFF Detect Player and turn ON Detect Subgrid.
(b) Set your ranges for Top / Bottom / Left / Right to 0.5m. Set Front / Rear as appropriate.
(c) Assign the Detect action to the Drop Item command on your group of dropper connectors.

Toggle the sensor on and off to enable / disable seed drop functionality.

The whole point is to give the sensor a precise point to lock onto with distinct deadzones between triggers. About the best you're going to do with large-grid is an Interior Pillar.

You could also just ghetto rig this another way using large-grid small inset connectors. LIterally just attach a small-grid small inset connector and battery with a control panel on it. Drop it dead-center on the large-grid connector and connect it. The power transfer will keep it from despawning, and you'e got your detection point. If you have to bypass server subgrid restrictions, this might be your best option. If you don't want the performance impact (or attention from server admins) of keeping it constantly connected, you could use an event controller to automatically cycle the connector depending on battery status so it only reconnects to charge up (to keep it from despawning). That only works if server Trash Removal isn't set to remove powered grids. But the whole idea is just to give you a precise detection point, ideally with the detection footprint of a single small-grid block.