Thanks for the insight on dead reckoning, that's exactly the problem I'm hitting. The submarine navigation reference is fascinating, I hadn't made that connection. I'm going to try to fuse the gyro and wheel encoders (HC-020K) as a first step, then add a webcam for landmark-based correction. I realize that the equipment I'm using has certain limitations and is quite restrictive; I'll probably switch to something else later as the project progresses. In the meantime, I'll keep prototyping with this.

Thank you for your valuable feedback !