Running my first test with the tomato made me quickly realise that I will need some kind of motor module to move up the arm that holds the light modules as the plant grows otherwise you will have to adjust the light position every other day.
I already have a design in mind, first I thought about adding a threaded rod in the middle but that makes keeping electrical contact with the rod a bit hard as the fuse holder can easily grip into the threading and either make the module stop or jump of the rod and loose connection.
Instead I will leave it as is and use a clamping mechanism to press motor with a U wheel bearing on its shaft against the middle rod with a second U wheel bearing pressing from the other side. Somewhat similar to how some 3D printers grip onto and move filament into the nozzle.
I'm not sure on the motor style yet. I don't think I want to go for high precision in any way but having +/-1mm precision would be nice because then you could use this platform for really large cable free CNC portal setups! Think about it, the motors can get control signals and power over the rods that give the whole thing structure. You could easily built a few meter long setups on your ceiling without needing a huge cable track moving along with it. I think that could be really powerful.
So I was thinking of either using small stepper motors or drone BLCDs, they are pretty cheap, small and have high torque, potentially moving heavier loads quite quickly. With very few steps they lack precision but you can use encoders to mitigate that and actually make them really precise. Though I will probably go with a more low cost sensor solution to stick with the style of the project. If it doesn't work out I'm fairly confident that standard steppers will do the work just fine.