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veloviewer explorer plugin
gpxeditor.co.uk is my current one, you can do some reasonable stuff with the free version but I cough up the £20/year to allow me to do some good VV route planning on it. nothing like the VV explorer plugin for Strava though.
I've posted upthread how I convert the unexplored tiles kml file to gpx and upload that so I can make sure my routes visit the unvisited tiles I want to cover. Doesn't have heatmap stuff but that's easily checked elsewhere.
(The developer is also over on yacf and he's responded to some of my queries, removed limits, improved code that hadn't assumed someone would upload a gpx file with thousands of routepoints, etc).
Finding another half-decent route builder is a bit of a project, especially with the veloviewer explorer plugin only supporting the current Strava route builder.