VVGmap

Works well and very easy to use. Use it on Edge 530 (also works with my old Edge 1000 explorer). Particular useful to recognise an upcoming tile-border. Tiles are best visible at lower zoom level, but that is less practical for navigation. (fiddly on 530 to change the zoom level, may be easier on devices with touchscreen). The grey line blends in with paths/streets, so less useful in urban areas.

Just found this extension. Very cool - I didn't realize the Edge's had been updated to handle this sort of overlay. This was a pretty smooth process (once I found my buried USB C adapter and microUSB data cable). Next step is to see how it works on the road, but I did a quick Browse Map on my Garmin Edge 530 map and I see tiles now!