I would especially recommend stitching vias where traces/buses intersect, especially when they do so at a large distance from any other return connection. Three near the crossing is the minimum to connect all four sides together (note that each bus cuts a slot in each layer, so there are four segments in question, at least locally), and four has nice symmetry.
The critical part is having a consistent impedance environment for a signal (trace) running over ground plane / within ground pours. The amount of consistency required is actually almost irrelevant, on a board this size, and using normal LVCMOS speed signals that most MCUs are capable of, for signal quality purposes -- but the concepts are what's important, and this has real consequences for EMC even into lower frequencies.
So, the important parts, don't cross a trace over a slot in the plane (which is why bus crossings require vias), keep the impedance reasonably similar (basically to say, keep the trace width, height over / distance to ground similar over the route), avoid large stub lengths, ensure short and wide ground-return paths (again, avoid slotting up GND!). And avoid islands, either by stitching them to GND, or removing them by area or lack of connectivity.