I like that you are implementing closed-loop control. This is all the rage also in large-scale heliostat fields. Most traditional heliostats are controlled using open loop, which places very strict requirements on both the mechanical structure, the actuators, and on the kinematic model, leading to expensive and very stiff heliostats. People are therefore moving towards cheaper heliostats where the tracking precision is achieved through closed-loop control. Implementing closed-loop control is a little bit more tricky when you have overlapping focal spots from thousands of mirrors, but there are approaches that are being developed, e.g. having cameras around the target looking back out over the heliostat field (developed by Heliogen among others).
You mention the challenge of light only being focused for a few hours per day. This is also a problem with large helisotat fields, and is also a field of active research. There's a group at University of Arizona with Professor Roger Angel developing heliostats that actively deform through the day to keep the perfect shape, and there's also an Australian company (Heliosystems) building heliostats that passively deform from gravity to keep as correct shape as possible.
When you are only using a single heliostat, as in your project, you could also consider building it as a Scheffler reflector - placing it on a single-axis polar-aligned tracking axis that passes through your target. Then it only requires single-axis tracking through the day, with some (possibly manual) seasonal adjusting.
I am very happy to see that you are highlighting the inherent risks in concentrated sunlight. There are lots of stories about people accidentally settings stuff on fire if the tracking doen't track correctly.